A Feather will let any racer who is equipped with it do a high jump into the air (and if that's not impressive enough, they also do a midair spin!) It's a great recovery Item when you're driving on a track with lots of edges and shortcuts that you can hop over that were otherwise inaccessible without a Feather. However, it is a dangerous Item to use on courses that have barely any railing and large areas that you can fall off the racetrack, such as Bowser Castle 3 (SNES). So you gotta be careful not to screw up and go flying off the edge of the course.
Feathers can also be used defensively against Items such as Green Shells and Bananas. You can simply evade an incoming Green Shell (or a Red Shell) by hopping over them. And Bananas are no longer a problem, cause now you can jump!
The item is almost the same. Only usable in battle mode. It can be used the steal a balloon in Balloon Battle, the Shine in Shine Thief, and some coins in Coin Runners. It can make you have a spin jump.