Jabber is the popular and original name of a chat protocol later known as XMPP. It is structured similar to e-mail but with a focus on supporting instantaneous communications and presence notifications. Unlike electronic mail, which was originally designed to work with disconnected servers, Jabber assumes that servers are always available, although users might not be. In practice, as Internet access has become nearly universal in the developed world, this is also the case for electronic mail.
Jabber is based on XML, which some consider a blessing in that it is readily extensible (via JEPs) while others consider one of its top two failings given its verbosity and weight when compared to lighter binary protocols. The centralized nature of Jabber and lack of multi-casting are also considered deficiencies in the protocol.
Jabber, in part of its open and standardized nature, has become a popular protocol for companies implementing their own chat system. It is the platform upon which Google, Live Journal, and Facebook.com have built their chat systems upon.