XMPP, which stands for Extensible Messaging and Presence Protocol, is a communications protocol for messaging systems. It is based on XML, storing and transmitting data in that format. It is used for sending and receiving instant messages, maintaining buddy lists, and broadcasting the status of one's online presence.
The protocol's original name was Jabber, and it was the basis of an instant messaging program with the same name. While the protocol was renamed, the Jabber instant messenger still exists today and is widely-used.
XMPP is an open protocol standard. Anyone can operate their own XMPP service, and use it to interact with any other XMPP service. The standard is maintained by XSF, the XMPP Standards Foundation.