A softphone is a software program for making telephone calls over the Internet using a general purpose computer, rather than using dedicated hardware. The softphone can also be installed on a piece of equipment such as a workstation, portable computer, tablet or even a cellphone and allows the user to place and receive calls without requiring an actual telephone set. Often a softphone is designed to behave like a traditional telephone, sometimes appearing as an image of a phone, with a display panel and buttons with which the user can interact. A softphone is usually used with a headset connected to the sound card of the PC, or with a USB phone.
To communicate, both end-points must support the same Voice-over-IP protocol, and at least one common audio codec. Many service providers use the Session Initiation Protocol (SIP) standardized by the Internet Engineering Task Force (IETF). Skype, a popular service, uses proprietary protocols, and Google Talk leverages the Extensible Messaging and Presence Protocol (XMPP).