Most common use - Jabber is a proggy that helps ichat connect to other IM's on the Mac. For example, I use Jabber via Ichat to get on to Gtalk (Google Talk). ON PC the common host IM software is Trellian.
Official definition - Jabber is best known as "the Linux of instant messaging" -- an open, secure, ad-free alternative to consumer IM services like AIM, ICQ, MSN, and Yahoo (see the IM quickstart). Under the hood, Jabber is a set of streaming XML protocols and technologies that enable any two entities on the Internet to exchange messages, presence, and other structured information in close to real time.
Here >
http://www.jabber.org/software/clients.shtml
Sorry for the shitty explaination