IRC (Internet Relay Chat) is a protocol that facilitates communication in the form of text. It is designed for group communication in discussion forums, called channels, but also allows one-on-one communication via private messages.
When you connect to an IRC network, you're connecting to a server that's part of that network. This server relays messages to and from other users connected to the same network.
The basic unit of communication on IRC is a message, which is a single line of text. Messages can be sent to a specific user (private message), to a group of users in a channel (public message), or to the server as a command.