Internet Relay Chat (IRC) is an instant Messaging service that has been around for a long time. It is old-school and not as user friendly as more modern IM services. The main difference is that with IRC, you join a channel and then can chat only with whoever happens to be online at the time. Users of the service will normally join the channel and then keep it in the background. It is recommend that you read up on how to use IRC before jumping in.
There are numerous different programs available to use IRC:
First Strike has a web based client set up with a First Strike Channel.