IRC (Internet Relay Chat) provides a way of communicating in real time with people from all over the world. It consists of various separate networks (or "nets") of IRC servers, machines that allow users to connect to IRC. The largest nets are EfNet (the original IRC net, often having more than 32,000 people at once), Undernet, IrcNet, DALnet, and Newnet.
Generally, the user (such as you) runs a program (called a "client") to connect to a server on one of the IRC nets. The server relays information to and from other servers on the same net. Recommended clients: