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.
The XMPP Standards Foundation (XSF) develops extensions to XMPP through a standards process centered around XMPP Extension Protocols (XEPs). The process is managed by the XMPP Extensions Editor and involves intensive discussion on the Standards mailing list, formal review and voting by the XMPP Council, and modification based on implementation experience and interoperability testing. All documents in the XEP series are available under a liberal IPR Policy for wide implementation. Submissions are welcome (see also the "inbox"). All XEPs and related files are under source control, old versions are available, and IETF-style XML reference files are provided. A compressed archive of all current XEPs can be downloaded here. You can view and submit XEP-related bugs and feature requests at the issue tracker.
This page lists approved XMPP extensions as well as proposals that are under active consideration. A list of all XEPs (including retracted, rejected, deprecated, and obsolete XEPs) is also available. Good places for developers to start are the client compliance and server compliance definitions, as well as the technology overview pages.