Super Mushrooms will make Mario grow bigger once eaten. It is one of the most common of items, just after the coin. If Mario touches an enemy while benefitting of the power of the Super Mushroom, he will shrink back to his small size. Also, if Mario takes a Mega Mushroom, when he shrinks back, he will be in his Super form.
Once Mario eats a Fire Flower, his suit and hat changes color, and he will be able to shoot fireballs.
Once Mario eats a Ice Flower, his suit and hat changes colour and he will be able to shoot iceballs which can freeze his enemies, which can be in turn used as projectiles, having the same effect as Green Shells.
This item was first introduced in New Super Mario Bros., and has since appeared in various other games. It has the power to make Mario supersized, and lets him leave a level in ruins after just walking through it.
After running out of power, Mario returns to his super form, regardless what he was before.