XMPP/Jabber in general
It is mainly used for Instant Messaging and the basis for many modern apps like WhatsApp, Facebook Messenger or Google Hangouts.
Being an open internet standard means that anybody can run a server that exchanges messages in a decentralized network, similar to e.g. email servers.
The standardization of XMPP Extension Protocols (XEPs) is the responsibility of the XMPP Standards Foundation (XSF) and is outlined in XEP-0001. These XEPs greatly improve the features and usability of the XMPP core protocol.