IRC stands for Internet relay chat. It is a kind of online chat room system where people can talk even when they are in different locations. It is a bit like a telephone conference line but with a text interface.
Technically IRC meetings are held every Monday evening at around 8pm onwards, but in general you can turn up at any point and there will be a bunch of people around - there are a handful of members who are regularly on from work as the channel can be both a useful resource and way to let off steam.
Some of the common people on channel are listed on Who's Where?
We hang out on #ALUG on the http://www.oftc.net/ OFTC network; this can be found by pointing your IRC client to irc.oftc.net (or alternatively irc.alug.org.uk, which points to OFTC).
Channel magnet link: irc://irc.oftc.net/#alug
IRC clients
Popular irc clients include Irssi (apparently the channel's text mode client of choice) and X-Chat. Here are a few links to client to get you started.
Unix
Irssi http://www.irssi.org/
X-Chat http://www.xchat.org/
Konversation http://konversation.kde.org/
BitchX http://www.bitchx.org/
EPIC - Enhanced Programmable ircii Client http://www.epicsol.org/
http://freshmeat.net/ (lots of them)
Mac OS X
Colloquy http://colloquy.info/
MacIrssi http://www.g1m0.se/macirssi/
Ircle http://www.ircle.com/ (shareware)
Should also run the Unix clients above.
Windows (if you must)
X-Chat 2 for Windows http://www.silverex.org/ (unofficial free port)
BitchX http://www.bitchx.org/ (ported from *nix version)
Irssi http://www.irssi.org/
mIRC http://www.mirc.com/ (shareware)