How it is used in Mario Kart
The mushroom is an item that gives a major boost that lasts about 1 1/2 seconds. You would normally find this item when you are in 3-4 place.
History
In new, modern Mario games like Super Paper Mario the mushroom would restore about 10 health.
The mushroom has also been used in Super Mario Kart, Mario Kart 64, Mario Kart:Double Dash!, and Mario Kart DS