It's simply a chat room. There are two parts. Client and server.
Client and server have their own "protocol" to communicate but user doesn't care. User uses the GUI to send and receive messages.
Server waits for any client to connect and then starts a new thread to deal with the client.
Client connects to server and can send and receive public or private messages with the other users.
Java chat ( Server and Client) can be downloaded from http://andpapad.googlepages.com or click here.
If you don't know how to run it just unrar - compile - run server - run as much clients as you want.
If you want to ask something just post it here and i will try to answer it.