Extensible Messaging and Presence Protocol (XMPP) (formerly known as Jabber) is an open sourced, XML-inspired protocol for near real time, extensible instant messaging (IM) and presence information (a.k.a. buddy lists). The protocol is built to be extensible and other features such as Voice over IP and file transfer signaling have been added.
Unlike most instant messaging protocols, XMPP is based on open standards. Like e-mail, it is an open system where anyone who has a domain name and a suitable Internet connection can run their own Jabber server and talk to users on other servers. The standard server implementations and many clients are also Free Software.