blocks|key|252280|text|API是Application+Programming+Interface的缩写，即API是应用程序与特定系统/应用程序/库等进行交互的方式。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|252281|例如，有用于操作系统(WinAPI)的API，用于其他应用程序(如数据库)和特定库(例如图像处理)的API，等等。|252282|API通常以客户端应用程序可使用的形式开发。对于C/C%2B%2B应用程序，它设置了头文件和动态/静态库。对于Java+-+jars集。诸若此类。|252283|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|I|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|G|$]]

API stands for Application Programming Interface, i.e. API is the way for an application to interact with certain system/application/library/etc.

For example, there are API's for OS (WinAPI), API's for other applications (like databases) and for specific libraries (for example, image processing), etc.

APIs are usually developed in a form consumable by a client application. For C/C++ applications, it a set header files and dynamic/static libraries. For Java - set of jars. And so on.

blocks|key|2937762|text|API是一个接口，通过它你可以访问别人的代码，也可以通过别人的代码访问你的代码。实际上是公共方法和属性。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2937763|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

An API is the interface through which you access someone elses code or through which someone else's code accesses yours. In effect the public methods and properties.

blocks|key|2937803|text|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2937804|应用程序编程接口(应用程序编程接口)是一组定义的函数和方法，用于与基本操作系统或在计算机上运行的另一个程序或服务进行交互。|ordered-list-item|2937805|它通常通过在软件中建立对库的引用或从dll导入函数来使用。|2937806|它在几乎所有软件中都以某种形式使用，在程序中显式调用或由编译器隐式调用。|2937807|2937808|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|-4|4|5|6|K|7|@]|8|@]|9|$]]|$1|A|3|B|4|C|6|L|7|@]|8|@]|9|$]]|$1|D|3|E|4|C|6|M|7|@]|8|@]|9|$]]|$1|F|3|G|4|C|6|N|7|@]|8|@]|9|$]]|$1|H|3|-4|4|5|6|O|7|@]|8|@]|9|$]]|$1|I|3|-4|4|5|6|P|7|@]|8|@]|9|$]]]|J|$]]

<ol>
<li>an API(Application Programming Interface) is a set of defined functions and methods for interfacing with the underlying operating system or another program or service running on the computer.</li>
<li>It is usually used by establishing a reference to a library in your software or importing a function from a dll.</li>
<li>It is used in one form or another in almost all software, being explicitly called in your program or implicitly called by the compiler.</li>
</ol>

blocks|key|3152312|text|搜索应该包括维基百科，它对一些编程概念/术语来说出人意料地好，比如Application+Programming+Interface|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|3152313|什么是？|style|BOLD|3152314|应用编程接口(+|3152315|+)是一组特定的规则(代码)和规范，软件程序可以遵循这些规则和规范来相互通信()。它充当不同软件程序之间的接口，并促进它们之间的交互，类似于用户界面促进人与计算机之间的交互。|blockquote|3152316|3152317|如何使用？|3152318|与使用任何规则集的方式相同。|3152319|何时何地使用它？|3152320|当然，这取决于领域和API。请考虑以下内容：|3152321|3152322|The+x86+(IA-32)+Instruction+Set+(非常有用；-)+BIOS+interrupt+call|ordered-list-item|3152323|OpenGL+WinAPI|3152324|The通常作为C库公开为|3152325|核心窗口系统调用:由浏览器向JavaScript|3152326|Web+services，公开的Ruby's+core+library|3152327|The+Document+Object+Model中的JavaScript|3152328|Web+services，类和方法，如Facebook's+Graph+API|3152329|An协议的实现所提供的类和方法|3152330|3152331|祝你编码愉快。|3152332|entityMap|0|LINK|mutability|MUTABLE|url|http://en.wikipedia.org/wiki/Application_Program_Interface|1|http://en.wikipedia.org/wiki/IA-32|2|http://en.wikipedia.org/wiki/BIOS_interrupt_call|3|http://en.wikipedia.org/wiki/OpenGL|4|http://en.wikipedia.org/wiki/Windows_API|5|http://en.wikipedia.org/wiki/Web_service|6|http://www.ruby-doc.org/core/|7|http://www.w3.org/DOM/|8|9|http://developers.facebook.com/docs/^0|X|X|0|0|3|1|0|0|8|V|0|0|0|5|0|0|0|8|0|0|0|0|V|1|15|J|2|0|0|6|3|7|6|4|0|0|0|0|C|5|G|J|6|0|4|L|7|0|0|C|8|J|K|9|0|0|0|0^^$0|@$1|2|3|4|5|6|7|25|8|@]|9|@$A|26|B|27|1|28]]|C|$]]|$1|D|3|E|5|6|7|29|8|@$A|2A|B|2B|F|G]]|9|@]|C|$]]|$1|H|3|I|5|6|7|2C|8|@]|9|@]|C|$]]|$1|J|3|K|5|L|7|2D|8|@$A|2E|B|2F|F|G]]|9|@]|C|$]]|$1|M|3|-4|5|6|7|2G|8|@]|9|@]|C|$]]|$1|N|3|O|5|6|7|2H|8|@$A|2I|B|2J|F|G]]|9|@]|C|$]]|$1|P|3|Q|5|6|7|2K|8|@]|9|@]|C|$]]|$1|R|3|S|5|6|7|2L|8|@$A|2M|B|2N|F|G]]|9|@]|C|$]]|$1|T|3|U|5|6|7|2O|8|@]|9|@]|C|$]]|$1|V|3|-4|5|6|7|2P|8|@]|9|@]|C|$]]|$1|W|3|X|5|Y|7|2Q|8|@]|9|@$A|2R|B|2S|1|2T]|$A|2U|B|2V|1|2W]]|C|$]]|$1|Z|3|10|5|Y|7|2X|8|@]|9|@$A|2Y|B|2Z|1|30]|$A|31|B|32|1|33]]|C|$]]|$1|11|3|12|5|Y|7|34|8|@]|9|@]|C|$]]|$1|13|3|14|5|Y|7|35|8|@]|9|@]|C|$]]|$1|15|3|16|5|Y|7|36|8|@]|9|@$A|37|B|38|1|39]|$A|3A|B|3B|1|3C]]|C|$]]|$1|17|3|18|5|Y|7|3D|8|@]|9|@$A|3E|B|3F|1|3G]]|C|$]]|$1|19|3|1A|5|Y|7|3H|8|@]|9|@$A|3I|B|3J|1|3K]|$A|3L|B|3M|1|3N]]|C|$]]|$1|1B|3|1C|5|Y|7|3O|8|@]|9|@]|C|$]]|$1|1D|3|-4|5|6|7|3P|8|@]|9|@]|C|$]]|$1|1E|3|1F|5|6|7|3Q|8|@]|9|@]|C|$]]|$1|1G|3|-4|5|6|7|3R|8|@]|9|@]|C|$]]]|1H|$1I|$5|1J|1K|1L|C|$1M|1N]]|1O|$5|1J|1K|1L|C|$1M|1P]]|1Q|$5|1J|1K|1L|C|$1M|1R]]|1S|$5|1J|1K|1L|C|$1M|1T]]|1U|$5|1J|1K|1L|C|$1M|1V]]|1W|$5|1J|1K|1L|C|$1M|1X]]|1Y|$5|1J|1K|1L|C|$1M|1Z]]|20|$5|1J|1K|1L|C|$1M|21]]|22|$5|1J|1K|1L|C|$1M|1X]]|23|$5|1J|1K|1L|C|$1M|24]]]]

Searches should include Wikipedia, which is surprisingly good for a number of programming concepts/terms such as <a href="http://en.wikipedia.org/wiki/Application_Program_Interface" rel="noreferrer">Application Programming Interface</a>:

What is an API?

<blockquote>
 <blockquote>
 An application programming interface (API) is a particular set of rules ('code') and specifications that software programs can follow to communicate with each other. It serves as an interface between different software programs and facilitates their interaction, similar to the way the user interface facilitates interaction between humans and computers.
 </blockquote>
</blockquote>

How is it used?

The same way any set of rules are used.

When and where is it used?

Depends upon realm and API, naturally. Consider these:

<ol>
<li><a href="http://en.wikipedia.org/wiki/IA-32" rel="noreferrer">The x86 (IA-32) Instruction Set</a> (very useful ;-)</li>
<li>A <a href="http://en.wikipedia.org/wiki/BIOS_interrupt_call" rel="noreferrer">BIOS interrupt call</a></li>
<li><a href="http://en.wikipedia.org/wiki/OpenGL" rel="noreferrer">OpenGL</a> which is often exposed as a C library</li>
<li>Core Windows system calls: <a href="http://en.wikipedia.org/wiki/Windows_API" rel="noreferrer">WinAPI</a></li>
<li>The Classes and Methods in <a href="http://www.ruby-doc.org/core/" rel="noreferrer">Ruby's core library</a></li>
<li>The <a href="http://www.w3.org/DOM/" rel="noreferrer">Document Object Model</a> exposed by browsers to JavaScript</li>
<li><a href="http://en.wikipedia.org/wiki/Web_service" rel="noreferrer">Web services</a>, such as those provided by <a href="http://developers.facebook.com/docs/" rel="noreferrer">Facebook's Graph API</a></li>
<li>An implementation of a protocol such as <a href="http://en.wikipedia.org/wiki/Java_Native_Interface" rel="noreferrer">JNI in Java</a></li>
</ol>

Happy coding.

blocks|key|2937888|text|它是一组相互交互的软件组件。它提供了一组函数、变量和对象类，用于创建应用程序、操作系统或其他任何东西。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2937889|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

It is a set of software components that interact with one another. It provides a set of functions, variables, and object classes for the creation of an application, operating system or any other thing.

blocks|key|257295|text|假设你正在开发一个游戏，你想让游戏用户在玩它之前登录他们的facebook档案(以获取你的档案信息)，那么你的游戏将如何访问facebook呢？现在来了API.Facebook已经为你写好了程序(API)，你只需要在你的游戏中使用这些程序application.using+Facebook-API你就可以在你的application.Here中使用他们的服务是一个很好的和详细的API…http://money.howstuffworks.com/business-communications/how-to-leverage-an-api-for-conferencing1.htm|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|257296|entityMap|0|LINK|mutability|MUTABLE|url|http://money.howstuffworks.com/business-communications/how-to-leverage-an-api-for-conferencing1.htm^0|5E|2R|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

Lets say you are developing a game and you want the game user to login their facebook profile(to get your profile information) before playing it,so how your game is going to access facebook?
Now here comes the API.Facebook has already written the program(API) for you to do it, you have to just use those programs in your game application.using Facebook-API you can use their services in your application.Here is a good and detailed look on API...
<a href="http://money.howstuffworks.com/business-communications/how-to-leverage-an-api-for-conferencing1.htm" rel="nofollow">http://money.howstuffworks.com/business-communications/how-to-leverage-an-api-for-conferencing1.htm</a>

blocks|key|3152372|text|API+1)什么是？|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|3152373|API是一种契约。承诺在以特定方式被要求时执行所描述的服务。|3152374|2)如何使用？|3152375|根据合同中规定的规则。API的全部意义在于定义它的使用方式。|3152376|3)何时何地使用？|3152377|当两个或多个独立的系统需要协同工作以实现无法单独完成的任务时，可以使用它。|3152378|entityMap^0|9|1|0|0|0|7|0|0|0|9|0|0^^$0|@$1|2|3|4|5|6|7|R|8|@$9|S|A|T|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|U|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|V|8|@$9|W|A|X|B|C]]|D|@]|E|$]]|$1|J|3|K|5|6|7|Y|8|@]|D|@]|E|$]]|$1|L|3|M|5|6|7|Z|8|@$9|10|A|11|B|C]]|D|@]|E|$]]|$1|N|3|O|5|6|7|12|8|@]|D|@]|E|$]]|$1|P|3|-4|5|6|7|13|8|@]|D|@]|E|$]]]|Q|$]]

1) What is an API?

API is a contract. A promise to perform described services when asked in specific ways.

2) How is it used?

According to the rules specified in the contract. The whole point of an API is to define how it's used.

3) When and where is it used?

It's used when 2 or more separate systems need to work together to achieve something they can't do alone.

blocks|key|2938006|text|好吧，除了所有的答案之外，我只是增加了一个例子。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2938007|正如其他人所说的那样，API+stands+for+Application+Programming+Interface通过哪个softwares+can+interact+with+each+other。注意，这不是人与人之间的互动。|offset|length|style|CODE|2938008|使用的|2938009|举个例子:你正在通过你的信用卡在线购买商品。您需要提供信用卡详细信息，然后按“继续”按钮。它会告诉你你的信息是否正确。为了提供这些结果，背景中有很多东西。|BOLD|2938010|该应用程序会将您的信用卡详细信息发送到远程应用程序，该应用程序将验证您的信息并将结果发送回您的应用程序。此场景使用的是API。|2938011|我希望它对那些不了解API是什么的初学者有所帮助。|2938012|另一个示例|2938013|天气应用程序|2938014|没有接口的-天气应用程序必须像人一样打开weather.com站点并读取详细信息。|2938015|带有接口的-天气应用程序将向weather.com发送一条消息并接收结果，然后显示结果。|2938016|源-各种在线资源|2938017|entityMap^0|0|B|1C|1R|12|0|0|0|25|0|0|1R|0|0|P|0|0|5|0|0|6|0|0|2|5|10|0|0|2|5|13|0|0|8|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|13|8|@$D|14|E|15|F|G]|$D|16|E|17|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|18|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|19|8|@$D|1A|E|1B|F|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|1C|8|@$D|1D|E|1E|F|L]]|9|@]|A|$]]|$1|O|3|P|5|6|7|1F|8|@$D|1G|E|1H|F|L]]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1I|8|@$D|1J|E|1K|F|L]]|9|@]|A|$]]|$1|S|3|T|5|6|7|1L|8|@$D|1M|E|1N|F|L]]|9|@]|A|$]]|$1|U|3|V|5|6|7|1O|8|@$D|1P|E|1Q|F|L]|$D|1R|E|1S|F|L]]|9|@]|A|$]]|$1|W|3|X|5|6|7|1T|8|@$D|1U|E|1V|F|L]|$D|1W|E|1X|F|L]]|9|@]|A|$]]|$1|Y|3|Z|5|6|7|1Y|8|@$D|1Z|E|20|F|L]]|9|@]|A|$]]|$1|10|3|-4|5|6|7|21|8|@]|9|@]|A|$]]]|11|$]]

Well, in addition to all the answers, I am just adding an example.

As others said <code>API stands for Application Programming Interface</code> through which <code>softwares can interact with each other</code>. Note, not a human interaction.

Where it is used

An example: You are buying an item online through your credit card. You will provide credit card details and press 'continue' button. It will tell you whether your information is correct or not. To provide these results, there are lot of things in the background.

The application will send your credit card details to a remote application which will validate your information and send the result back to your application. API is used in this scenario.

I hope it helps for the beginners who don't understand really what API is.

ANOTHER EXAMPLE

Weather application 

Without API - Weather application must open weather.com site and read the details as a human does. 

With API - Weather application will send a message to weather.com and receive the result and then display it.

SOURCE - Various online resources

blocks|key|3152494|text|API定义了一个软件在源代码级别与另一个软件进行通信所使用的接口。它通过提供一组标准的接口(通常是函数)来提供抽象，一个软件部分(通常是较高级别的部分)可以从另一个软件部分(通常是较低级别的部分)调用这些接口。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3152495|例如，API可能通过提供绘制文本所需的一切的一系列函数来抽象在屏幕上绘制文本的概念。API仅定义接口；实际提供API的软件称为API的实现。|3152496|将API称为“契约”是很常见的。这是不正确的，至少在法律意义上是不正确的，因为API不是双向协议。API用户(通常是较高级别的软件)对API及其实现没有输入。它可以按原样使用API，也可以根本不使用它:接受或放弃！|3152497|API的一个实际示例是由C标准定义并由标准C库实现的接口。此API定义了一系列基本和必要的函数，如内存管理和字符串操作例程。|3152498|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|L|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|M|8|@]|9|@]|A|$]]|$1|H|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|I|$]]

An API defines the interfaces by which one piece of software communicates with another at the source level. It provides abstraction by providing a standard set of interfaces - usually functions - that one piece of software (typically a higher-level piece) can invoke from another piece of software (usually a lower-level piece). 

For example, an API might abstract the concept of drawing text on the screen through a family of functions that provide everything needed to draw the text. The API merely defines the interface; the piece of software that actually provides the API is known as the implementation of the API.

It is common to call an API a "contract". This is not correct, at least in the legal sense of the term, as an API is not a two-way agreement. The API user (generally, the higher-level software) has zero input into the API and its implementation. It may use the API as-is, or not use it at all: take it or leave it! 

A real-world example of an API is the interfaces defined by the C standard and implemented by the standard C library. This API defines a family of basic and essential functions, such as memory management and string manipulation routines.

blocks|key|252434|text|用外行的话来说，我一直说API就像是两个说不同语言的人之间的翻译器。在软件中，可以使用API+(或转换器)使用或分发数据，以便两种不同类型的软件可以通信。好的软件有一个强大的转换器(API)，它遵循安全和数据整洁的规则和协议。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|252435|我是一个市场营销人员，不是一个程序员。这一切可能都不太正确，但这是我10年来一直试图表达的……|252436|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

In layman's terms, I've always said an API is like a translator between two people who speak different languages. In software, data can be consumed or distributed using an API (or translator) so that two different kinds of software can communicate. Good software has a strong translator (API) that follows rules and protocols for security and data cleanliness. 

I"m a Marketer, not a coder. This all might not be quite right, but it's what I"ve tried to express for about 10 years now...

blocks|key|2938039|text|API是一组命令、函数和协议，程序员在为特定操作系统或任何其他软件构建软件时可以使用这些命令、函数和协议。API允许程序员使用预定义的函数与操作系统交互，而不是从头开始编写它们。诸如Windows、Unix和Mac之类的所有计算机操作系统和诸如Java之类的语言都为程序员提供了应用程序接口。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2938040|Source|offset|length|2938041|entityMap|0|LINK|mutability|MUTABLE|url|http://techterms.com/definition/api^0|0|0|6|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@$D|P|E|Q|1|R]]|A|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|M]]]]

An API is a set of commands, functions, and protocols which programmers can use when building software for a specific OS or any other software. The API allows programmers to use predefined functions to interact with the operating system, instead of writing them from scratch.
All computer operating systems, such as Windows, Unix, and the Mac OS and language such as Java provide an application program interface for programmers.

<a href="http://techterms.com/definition/api" rel="noreferrer">Source</a>

blocks|key|3152529|text|应用程序编程接口()是一组用于构建软件应用程序的例程、协议和工具。API指定软件组件应该如何交互，以及在对图形用户界面(GUI)组件进行编程时如何使用API。一个好的API通过提供所有的构建块使开发程序变得更容易。然后，程序员将这些块放在一起。|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|3152530|来源：http://www.webopedia.com/TERM/A/API.html|3152531|entityMap|0|LINK|mutability|MUTABLE|url|http://www.webopedia.com/TERM/A/API.html^0|0|9|0|3|14|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@$9|Q|A|R|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|S|8|@]|D|@$9|T|A|U|1|V]]|E|$]]|$1|H|3|-4|5|6|7|W|8|@]|D|@]|E|$]]]|I|$J|$5|K|L|M|E|$N|O]]]]

Application program interface (API) is a set of routines, protocols, and tools for building software applications. An API specifies how software components should interact and APIs are used when programming graphical user interface (GUI) components. A good API makes it easier to develop a program by providing all the building blocks. A programmer then puts the blocks together.

source:
<a href="http://www.webopedia.com/TERM/A/API.html" rel="nofollow">http://www.webopedia.com/TERM/A/API.html</a>

blocks|key|256429|text|解决这种情况：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|256430|马克和丽莎是一对秘密情侣，由于年龄不同，他们不能在一起。马克和丽莎每晚都会在没人看的时候见面。他们已经建立了自己的一套规则，当时机到来时，如何进行沟通。他站在她的花园里，向她的窗户扔了一块小石头。丽莎知道是时候了，她的反应是从窗户挥手，然后打开窗户，这样马克就可以爬进去了。这就是API如何工作的示例。岩石是对另一端的初始请求。另一端挥舞着，打开了窗口，基本意思是“欢迎进来！”|256431|API几乎类似于人类的语言，但适用于计算机。|256432|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|I|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|G|$]]

Conaider this situation:

Mark and Lisa are secretly a couple, and because of age difference they are not allowed to be together. Mark and Lisa meet every night when nobody is watching. They have estabilished their own set of rules how to comunicate when the time comes. He stands in her garden and throws the small rock at her window. Lisa knows that it is time, and responds by waving from the window and opening it afterwards so Mark can climb in. That was example how the API works.
The rock is initial request to another end. Another end waves, opens the window which basicaly means "Welcome in!". 

API is almost like human language but for computers.

I've searched for the definition of an API in a programming language and I am still finding it hard to understand.

Could anyone advice me in simple, layman's terms:

<ol>
<li>What is an API?</li>
<li>How is it used?</li>
<li>When and where is it used? </li>
</ol>

What exactly is the meaning of an API?

图像处理

内存管理

数据库

编译器

Windows

Java

Unix

我已经在编程语言中搜索了API的定义，但我仍然发现它很难理解。有没有人能用简单、外行的话给我一些建议：什么是API？如何使用？何时何地使用？

问API的确切含义是什么？
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