The Spin Attack is Link's (in both SSB and SSBM) and Young Link's Up B move.
The move involves Link/Y. Link spinning his sword around in a circle.
SSB
Link's Spin Attack has identical animation, knockback, and damage when used on the ground and in the air. Both versions deal substantial knockback, making them good combo finishers. As a recovery move, though, it is lacking in both horizontal and vertical range.
SSBM
If used on the ground, Link will have a bit more range and opponents will be hit away quite a distance. Young Link's version has less range, but will suck in opponents to deal several hits of damage. If used in mid air, the move is the same for both Link and Young Link, they will spin while moving upward, dealing a possible 3 hits, the third having considerable knockback to it.
Spin slashes have been a standard move in Zelda games, it's first appearance being Legend of Zelda: A Link to the Past. While usually a basic move available from the start, in some games it can be upgraded with magic to deal more damage and have a greater range.