In order to join any of these channels, you must have an IRC client installed. A good IRC client that's available on Windows, OSX and Linux is Xchat. You are also able to join through a web service, such as Mibbit; although standalone clients are often said to provide a better experience.
More information on IRC can be found on Wikipedia.
- #Minecraft - The original channel made by Notch, housing between 500 and 600 users each day. Discussion on this channel is not always centered around Minecraft as the channel serves as a place for Minecraft's community to just talk about whatever they want (as long as it doesn't break any rules). Generally anybody is welcome here granted they follow the channel's rules.
- #Minecrafthelp - The main support channel. Players who seek help with Minecraft are able to ask for support here and discuss any problems they may have.