IRC (Internet Relay Chat) is a chat protocol which allows for chatroom-style online conversations. It can be accessed by using an IRC client such as HexChat or mIRC, or by using a web client in your browser such as Freenode's WebChat.
Our IRC channel is linked to our Minecraft server, so it's possible to chat with people on the server without actually being logged in. This also allows ops to run server commands through IRC without needing to be online.
We are also running an instance of the Willie IRC bot, which allows for certain extra functionality. To see a list of commands, type .commands into the IRC channel.