I the basic, Jabber is just another chat network. It means that you can add contact to chat to. The other contact also needs to have Jabber*. Since a few days, Google opened s2s (server to server) to their server, so now are all Google Talk users also Jabber users.
Besides private conversations, it is possible to make Multi User Chats (MUC). Multi User Chats are comparable with an IRC channel. MUC is the successor of conference (I don't know the exact name). I don't know if they are both supported in your version of Kopete. Searching for MUC and Jabber leads to some more information about this.
I put a star at the 3de centence. There is a way to speak with people using a different protocol. Therefor, you must find a server which have a YAP-transport (YAP is just a name for another protocol, such as AIM, ICQ, ... ). That server makes contact with your server, and with the server of the other network. You can leave your login information at that server, and then you can chat with the users on that network. For more information, searching for Jabber Transport is proberly sufficient. I don't know if Kopete can this, because in Kopete it is easier to connect directly to that server. Also, if you use a transport, you only have the features that both protocols have.