The Cape is a magical item first appearing in Super Mario World as well as its Game Boy Advance remake. Mario and Luigi gain it upon touching a Cape Feather (an item usually found in ? Blocks), and it gives them the ability to fly, among other things.
In Super Mario World 2: Yoshi's Island and its Game Boy Advance remake, Baby Mario could get a cape and become invincible by touching a star; Yoshi, upon doing this, would turn into a Yoshi Egg and follow with the rest of the eggs.
The cape also makes an appearance in Super Mario Advance 4: Super Mario Bros 3. It could only be obtained by using an e-Reader card to scan the feather in to the player's item inventory. The feather and in turn the cape was also found in the Japan-only World-e level Henshin Mario de Tsukisusume!!. The cape has the same effects as it does in Super Mario World.
Mario later uses it as an attack in Super Smash Bros. Melee and Super Smash Bros. Brawl. It is his Side + B move. While this attack is not particularly powerful, it is useful in that it can deflect certain projectile attacks. It can also make the foe spin around, and face the other direction.
In the comic series Super Mario Adventures, the cape is used multiple times by the protagonists in the story. The cape is first seen to being used by Peach who uses the cape to escape from Bowser's Castle to get away from the tormenting of the Koopalings. The cape is later used by Toad to scout out for assistance in order to help himself as well as Mario, Luigi, Yoshi, and Peach find a way back home to Peach's Castle after having defeated Bowser and his forces.