Edit: I really must stop being so verbose in my replies.
Why not.
IRC aka internet relay chat is a chatroom protocol that has been around for years (far more than even ICQ).
MIRC is one of the most popular programs to use when chatting upon IRC to the extent the brand name becomes the term used (although many would be far quicker to discourage the use of MIRC to mean IRC than perhaps hoover, biro, rizla, flannel, bic.....).
There are many other programs but I will leave those for another day (MIRC is the most popular for a reason although the others are damn good).
As a sidenote the two most popular DS irc clients are probably DSOrganize (good for quick joins and minor chat) and Clirc (sometimes written as c|irc (the | being the symbol left of the Z on a European keyboard, I forget where it is on a US keyboard but I think it may be next to the 1 key)) which is a full fledged client up there with the top PC apps in my opinion.
There are irc servers (actually there can be multiple servers all over the world but running one "server") which then have chatrooms in them. Some are general purpose servers like efnet, rizon, irchighway while others tend to only house the chatroom for a site (float around various web service providers and you will eventually find one).
Some servers go a stage further and add nick (the name you use when chatting) and channel (the chatroom itself) registration (see nickserv for nicks and chanserv for channels) and virtual hosting (anyone can find out your IP/host name and some clients even display it when people join) but not efnet (too big or something).
As these are usually geared towards multiple user chat various people with more power can appear (who and why is much like any forum), broadly speaking there are full ops, half ops (sometimes) and voiced users above the normal users (there are levels among those as well on occasion but I will leave that for another day). Full ops can make all changes and kick/ban who they like, half ops can usually do that but not to other ops and voiced users generally only have the ability to speak when the channel is silenced to normal users (I forget the exact term). Voiced users may also be able to access command in channels with bots that normal users can not.
Tutorial, I think it was sinkhead who made the tutorial and it jph who made the topic on the frontpage a few days back.
