Somersault is a move used to reach high places without moving, though it does take a little time to use. The character flips over a couple of times and you can chain it to a Wall Jump or Ground Pound.
So far only Mario, Luigi, Wario, and Yoshi can use this move. Although Luigi is the only one can Spin Jump on the way down from a Somersault.
It was introduced in Super Mario 64. In Super Mario 64 DS the rest of the characters could use it. Also in Super Mario Sunshine Mario sprays a blast of water during a Somersault.