no offence but.. wah!?!?! i didnt get a word of that
you can use tcp with sockets to connect client applets to a server application (threaded for multiple users)
or udp packets (but this is quite unreliable apparently)
or multicast to broadcast messages even if no1 is listening(not in an applet)
im makeing a chatroom for my ict coursework and im using threaded tcp sockets (its all cofusing me but it should work

)