WHAT IS JABBER
Jabber is the original open instant messaging (IM) technology, invented by Jeremie Miller in 1998 and formalized as the Extensible Messaging and Presence Protocol (XMPP) by the IETF as an Internet Standard for IM and presence.
For more information about XMMP please consider https://en.wikipedia.org/wiki/XMPP
USING YOUR JABBER ACCOUNT
After you registered an Jabber-account on Wusz.org, you can connect to this service with dozens of free IM clients that support the XMPP standard. Here is a list of some free XMPP clients on Wikipedia: https://en.wikipedia.org/wiki/Category:Free_XMPP_clients