IRC stands for “Internet Relay Chat”. It's a chat protocol used extensively at W3C.
The CSSWG's channels aren't particularly busy except during meetings or events, but they're a critical part of our communication tools. You're most likely to run into people during West Coast daytime / Europe evenings.
The main CSSWG channel is used by the CSS Working Group members during meetings and for quick coordination during off-hours. Topic is anything we work on. (Help with using CSS is off-topic.)
We also have a channel dedicated to CSS conformance testing. Here's the information you'll need:
You'll need an IRC client. Opera has one built-in. Mozilla has “Chatzilla”, which is part of Seamonkey and available as an add-on for Firefox. Pidgin and Trillian can also do IRC. There's a web interface to IRC called Mibbit (type 'irc.w3.org:6665' as the server name). And there are various stand-alone programs you can download.
IRC commands are prefixed with a slash. (Any other text you type into a channel gets posted as a message to the channel.) Some basic commands: