IRC is an internet set of text chat servers, that let people meet in #channels (groups or rooms) in order to interact.   You can lock out others if you wish, whisper to single players, and even play sounds and midi music.

That said, one can only imagine the fun you could have using this tool to game with.  Especially with friend that have moved away, new ones not in your area, and play game sessions from you own home that would have been impossible to play because of time.

My favorite IRC application is MIRC, for reasons of its sheer power.  With only a little scripting work, I made pop-up menus to call upon banks of sounds and music.  I also have written die rolling programs, so rolls are seen by all.

Other application that can be used are programs like Pirch, and  PowWow.  While these programs are a bit easier to operate at first, at  the time of this writing, they can't compete with the power of MIRC.  To be fair though Pirch is close, and PowWow has a wonderful white board feature.  There is also Homer and  Ircle for the mac.  I am sorry that I    don't know much about these.

While live gaming is the best.   The atmosphere can be really developed sitting alone at your PC and hearing eerie music, or a monster growl surprise you.  I have made a CD for our group that is packed with 400MB of sounds and midi files to choose from.  All with simple scrolling pop-up menus to make speedy choices.  If you haven't tried this then you are missing something.