IRC (Internet Relay Chat) is a text-based chat medium. In short, it is the predecessor to instant messaging clients most people use today. Although somewhat antiquated, it is still in use and is a great alternative to proprietary and closed-off chat media. The Adventure Time Wiki has an unregistered channel on the FreeNode IRC network; for IRC-savvy users, simply join on irc.freenode.net.
If you are unfamiliar with IRC, or are behind a firewall preventing you using it, visit http://java.freenode.net//index.php?channel=adventuretime, choose a name and click login. By default, you will join the Adventure Time channel. Other channels are available by clicking the advanced options link.