One of the first widespread chat protocols (RFC 1459), forming the biggest international chat network today, based on open standards (as opposed to proprietary stuff like AIM, MSN-IM or ICQ). Chat servers are networked together, with distributed traffic. Used today for lightweight chat, customer support, developer collaboration, etc.
http://www.irchelp.org/irchelp/rfc has more technical information.
See also: ChatLectures.
It is interesting to consider some of the [things] that have evolved out of the basic system:
How to use an IRC client:
There are two ChannelBots in #wiki, one being the requisite security bot, and thus ignorable. The other is much more interesting. DeepakGoel wrote wikibot originally for #emacs, but he has gloriously made it run on #wiki as well. It's very sexy, having already learnt all about MeatballWiki. As an example:
<Sunir> , InternetRelayChat <wikibot> InternetRelayChat --  at http://www.usemod.com/cgi-bin/mb.pl?InternetRelayChat
It can also search Google!
<Sunir> , "pet potato" <wikibot> [google] http://www.ccmuseum.org/KidsPages/potatobug.htm
See EmacsWiki:ErBot for more fun with sexy wikibot.
I love you wikibot!
the lag-monster usually tossed me into the net-split
Despite the popularity of other chat protocols like Discord and Matrix, I still prefer IRC. Its simplicity is too beautiful for it to die.
People say that not being able to remove past messages, not being able to retrieve messages that were posted while you're offline, as well as not being able to upload media to the server, are weaknesses of IRC. But I don't see them as weaknesses, but rather as its strengths.
How people are able to see the ability to censor past messages as a win is beyond me. Worried about spam? IRC already has good anti-spam measures, like Sigyn. And even then, if you don't want to see such messages, feel free to censor it for yourself only. But everyone else should have the choice to see them or not.
The problem of not being able to backtrack already has solutions. Bouncers (which are basically 24/7 IRC clients) lets you see messages that were posted while you're away. But what about those messages before you joined that channel? Chances are, there could be public logs of that channel which you can read. But what if there aren't? Maybe you should consider that there's a good reason for that. If a channel prohibits public logging, that's because they want to protect their users. And besides, you shouldn't concern yourself with the past, but the present (and future).
You can't upload binary data to the server itself, but you can upload them via DirectClientToClient?, or DCC for short. It's basically an extension to the IRC protocol which adds peer-to-peer capabilities. This is great, because the server doesn't have to suffer from liability of uploaded content. And have I said that this is a good example of abiding by one of the tenets of the UnixPhilosophy?, which is "do one thing and do it well"?
So IRC will be here to stay, at least for centuries, whether you like it or not. ;) -- JobBautista