Although Jabber is often discussed as if it were a single server or client, it is actually a set of streaming XML protocols and technologies containing a codebase from which all manners of instant messaging (IM) servers and clients are built. Jabber is a development platform for customizing and extending IM products adhering to the open and freely available Extensible Messaging and Presence Protocol (XMPP).
Jabber offers gateways for communicating with other IM networks: Yahoo! Messenger, ICQ, AOL Instant Messenger (AIM), and MSN Instant Messenger. This is subject to change, however, as the big commercial networks are notorious for trying to block access to users of the "wrong" messaging clients and other networks. But the Jabber maintainers do a good job of keeping up, and, of course, any ace programmer can make modifications.