IRC is Internet Relay Chat, as defined in RFC 1459. Despite being standardized in an RFC, most IRC implementations deviate in minor ways from the specification.
IRC is one of the oldest IM systems. Its RFC makes reference to its initial design work starting in 1989.
IRC is a TCP-based protocol, and is completely text-based. It's possible to use a standard telnet client as an IRC client if one knows the protocol well enough. IRC historically has been used mainly for chat rooms, in contrast with AIM, ICQ and others that have been primarily used for one-on-one messaging.
IRC servers usually run on TCP ports 6666 or 6667. File transfers take place over IRC-DCC (Direct Client-to-Client) on unpredictable ports. These connections can be tracked by Netfilter for purposes of making sure they get through its NAT or to identify them as IRC.