IGS is the internet go server. It always has hundrends of people online, playing the game go. This is the most popular out of many internet go servers. IGS allows free access to people out of japan, but blocks japanese ip addresses unless they pay for the priveledge. I think it's 3000 yen per month or 250 yen per game.
IGS broadcasts many pro games, and often has pros come on to play amateurs anonymously. It is fun to see one of these guys appear - they beat absolutely everyone, but no one knows who they are, although there is a lot of speculation.
IGS ranks run from 30 kyu to 9 dan In general, IGS ranks are stronger than any real world ranks.

IGS is run by a guy named tweet. For a long time when you logged on, there was a message saying "there is no expectation of privacy on this server". I didn't know what that meant until one of my friends got banned for mentioning another go server in a private message to another user. Apparently the administration of IGS greps all private messages for certain key words they don't like. They also ban people arbitrarily.
The address is igs.joyjoy.net 6969. It's a telnet protocol.

The official client program is panda-egg, which is pretty nice but is constantly having more ugly annoying features added.