IRC is a kind of chat protocol. It is focused towards group communication, yet allows one-to-one communication. Unlike Jabber, IRC users of one network/server can only communicate with channels and nicks of that network/server. In IRC realm, channels are group-chats (like MUC in Jabber) and nicks are user-identifiers (like JIDs in Jabber). To learn more about IRC, please visit IRC-Help.
Here, you'll see how to connect to IRC from your Jabber account, via an IRC transport for Jabber, called Biboumi.