Basically, whilst your Mario Kart Wii disc is safely stored in it's shiny white case, you can still download things like missions, ghost data, and eventually, possibly, tracks.

"We've known for a while that Mario Kart Wii would support twelve-player online races, but given Nintendo's less-than-stellar online record, we were a little worried about how smooth the online experience would be. Well, we're a little less worried now that the Nintendo UK web site has revealed some details of how online racing on the Wii will work. Among the details discussed:

* Spectator mode - When you join a room in progress, you can watch the race unfold while waiting for a new one to start.
* Global friend view - Lets you view your friends' locations and online status.
* Room structure - Host chooses race type, but player votes determine course selection.

The site also revealed a dedicated Mario Kart Wii channel, which can be run independently of the disc-version of the game. The channel lets players view global rankings, share and download ghost data, and download daily missions (similar to those found in Mario Kart DS). Can we suggest a mission dedicated to arranging dropped banana peels into jaunty patterns? That would be awesome."

Source.