It's a very plain and simple chat program that consists has channels which contain numerous users. One of the main features are that you can be in many chatrooms at once participating in many different conversations. It has been around longer since ICQ, MSN, AIM, Xfire, etc. and is where you can find most of the organized PB2 teams.
For an introduction to IRC and where to find the PB2 crowd, I'd check out section B-2 in
Lekky's Clan Guide.
