XMPP is the Extensible Messaging and Presence Protocol, a set of open technologies for instant messaging, presence, multi-party chat, voice and video calls, collaboration, lightweight middleware, content syndication, and generalized routing of XML data.

XMPP was originally developed in the Jabber open-source community to provide an open, secure, spam-free, decentralized alternative to the closed instant messaging services at that time. XMPP offers several key advantages over such services:

via XMPP Technologies Overview – The XMPP Standards Foundation.