DS-X2 as a nice compilation of all the features include in the different modes of multiplayer for Mario Kart, here is what the Wifi Mode will include.

• Nintendo Wi-Fi Connection:
• 2-4 Players racing simultaneously
• Choice of playing with:
  - Friends Only (players registered as Friends)
  - Rivals (players with similar skills)
  - Recent Opponents
  - Regional Racers (players in their country)
  - World Racers (players anywhere in the world)
• No limit on course choice, but each player votes for a course and the course with the most votes wins (in case of a tie, the course is chosen randomly)
• No limit on character choice
• Players can create and use personalized player icons

You can find what’s included in Download-Play and Multi-Mart play here.

Also, check out a few new shots of the game here.

source: DS-X2 & NintendOwn