Internet Relay Chat (IRC) is a form of real-time Internet chat or synchronous conferencing. It is mainly designed for group communication in discussion forums called channels, but also allows one-to-one communication via private message, as well as chat and data transfers via Direct Client-to-Client.

IRC was created by Jarkko Oikarinen in late August 1988 to replace a program called MUT (MultiUser talk) on a BBS called OuluBox in Finland. Oikarinen found inspiration in a chat system known as Bitnet Relay, which operated on the BITNET.

IRC gained prominence when it was used to report on the Soviet coup attempt of 1991 throughout a media blackout. It was previously used in a similar fashion during the Iraqi invasion.

Technical Information

IRC is an open protocol that uses TCP and optionally TLS. An IRC server can connect to other IRC servers to expand the IRC network. Users access IRC networks by connecting a client to a server. There are many client and server implementations, such as mIRC and the Bahamut IRCd, respectively. Most IRC servers do not require users to log in, but a user will have to set a nickname before being connected.

Networks 

There are thousands of running IRC networks in the world. They run various implementations of IRC servers, and are administered by various groups of IRC operators, but the protocol exposed to IRC users is very similar, and all IRC networks can be accessed by the same client software.

One can join servers by clicking on an irc://irc.network.example:port/channel hyperlink.

The largest IRC networks have traditionally been grouped in The Big Four — a designation for networks that top the statistics. The Big Four networks change periodically, but due to the community nature of IRC there are a large number of other networks for users to choose from.

The Big Four:

EFnet
IRCnet
QuakeNet
Undernet

Source: Internet Relay Chat - Wikipedia, the free encyclopedia