Note: This channel has to be loaded from the game initially, and can also be accessed via the game, but you can use the channel when the game disc is not in the Wii.
The channel has four main sections: Friends, Ghosts, Ranking, Tournaments.
With the friends channel, you can simply see if any of your friends are online, and if they are, you can join them or invite them to play.
With the ghosts, you can send and download ghosts as well as race some random ghost from some random track.
With the tournament section, you can join current tournaments, of which there are none, but the future is sure to hold some cool stuff.
Rankings is the most exciting section to me. Here, you can see who holds the top spot in three sections: friends, regional, and worldwide. Each shows a number line of the average times of players for a single track at a time and how you stack up, with the top 10 being able to be viewed. You are also given the ability to race the top man's ghost to see if you can keep up, which is pretty friggin' awesome, considering you can race against the #1 player in the world. Also, in worldwide ranking you can race against the Staff Ghost, which is always fun.
Overall, the Mario Kart Channel is pretty nifty, but will only really be used to check if your friends are online, seeing as how you can and will access the rest of this stuff via the game itself.