Internet Relay Chat, or IRC, is a protocol that allows users that connect to Internet Relay Chat Servers to have converstaions with others in real time. Users connect to IRC Servers using an IRC Client. There are many popular IRC clients available on the Internet for all operating systems. These clients are generally a graphical interface to the IRC Server which allows you a familiar interface to the chat system. There are pure text versions of the client found for Unix systems as well.
IRC Servers are set up as a network of many servers, in which each server allows connections from the user's client programs. Regardless of which IRC Server a user connects to on an IRC Network, they will be able to chat with any of the users connected to any of the IRC servers that are part of that particular IRC Network.