The game is a collection of various events based on the Olympic Games. Players assume the role of a Nintendo or Sega character while competing against the others in these events.
Several Olympic events on the Wii, including soccer, badminton, and equestrian debut alongside improved versions of previously existing events such as athletics, aquatics, and table tennis. The Wii game introduces new "Dream Events", which are alternate versions of Olympic events taking place in locations from older games of the Mario and Sonic series. The Wii version also introduces new cooperative mechanics and a Party mode.
The 3DS version will have over fifty Olympic-based events in single-player and multiplayer.
Source: Wikipedia, "Mario & Sonic at the London 2012 Olympic Games", available under the CC-BY-SA License.