There are several servers to play online backgammon, even for free.
This site is mainly about the First Internet Backgammon Server (F.I.B.S.).
To play on Fibs you must first register on the FIBS game server (not on this website).
You can do that by using this form or the client software you downloaded.
If you have registered and don't want to download any software you can click here to play now!
other servers include tigergammon (tigergammon.dyndns-home.com port 4321) and melbg (server.melbg.net, also port 4321) see here for more info on these newer setups