Jabber is a chatsystem. In many ways it resembles email. It's not intended for letters, but for chatting. Jabber is not a company and not a software program. Just like email is not a company or a software program.
See Also XMPP.
The program you use to connect to a Jabber server. Pandion is a Jabber client.
See Also Jabber server.
Jabber ID or JID is your address with which you are recognized on the Jabber network. It consists of a username, an @, and a server. A typical Jabber ID looks like: "[email protected]" (without the quotes).
A Jabber server is a server that offers Jabber. You can use a Jabber server with a Jabber client. The Jabber server is part of your Jabber UD; the part after the @.