Roster is a list of other Jabber users that you want to be associated with. You can see if they are online and click on their name to chat. Presence is your status online. You set it so other people can see if you are online, away, or whatever else you want. Messages and Chats are the same except for their GUI which is determined by the client.
Messages are usually for short notes, while Chats are for longer conversations.
Those are the big five right there. Of course there are other features, but you can learn those easily on your own. Now, go ahead and enter in the login information you want, and create your account on mozdev.org. It is now time to add some people to your roster. There should be a menu option for doing this.