IRC is Internet Relay Chat. I've used it quite a few times. Basically you log onto any of dozens of IRC servers all over the world, join a chat room (called a channel) or create one. The server sits there online 24/7 most of the time and relays whatever you type to everyone else logged onto the channel so that they see it in a matter of seconds, usually almost instantly. If t hings are busy it may take 3 or 4 seconds, but it's the next best thing to a phone call.
Most chat rooms can call IRC granddaddy, it was the original chat program back in DOS days. Now you can get chat programs with automatic smilies, PM (private messing, for the uninitiated), video, sound, profiles...just about anything you can think of that might be useful, someone has added it to a chat program. But they all started with IRC.
The basic difference is IRC requires a "client", like mIRC or those already mentioned, and the use of a few commands is occasionally helpful. And if you want to check it out and go join a channel, be sure and learn a bit about "netiquette". Some of the IRC old timers will ban you forever for being a jerk. Also watch out for trojans, NEVER accept any file sent to you by someone you do not know, especially someone who is not currently in the channel you are in. NEVER....can't stress that strong enough.
I used to accept them then zip them up and send them to a friend, also using a Linux box, who would open them up in a compiler to see how they ticked...the people sending them didn't know I was on a Linux machine, so Windows viruses and trojans would not run, or that they were immediately being sent to a programmer for inspection...I used a different nick every week or so and accumulated quite a collection of malicious files. For instance I had over 20 different copies of the Klez virus in a quarantine folder, 20 or 30 copies of Bad Trans, several others too, at least a dozen trojans, just about every spyware program known at the time...and no, none of them were EVER allowed to get out into the wild. I finally deleted them all...
The point is, be careful. IRC is a great chat platform, but it does have a rather large dark side, constantly searching out the uninformed...do not ever accept any file from a stranger.
EDIT: Forgot to add, this is the link to mIRC, one of the oldest and very popular IRC client
http://www.mirc.com/
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