From the Super Mario Wiki
The Metal Cap is an accessory that makes its debut appearance in Super Mario 64. It is usually found in Metal Boxes. The Metal Cap can turn Mario into Metal Mario, which makes him invulnerable and extremely heavy. The side effects of this temporary invulnerability are that he can no longer make very high or long jumps, cannot swim, and will sink faster in quicksand. Fortunately, while in metal form, the player is also unaffected by toxic fog, and cannot drown; they also have access to extremely powerful Ground Pounds.
If Mario were to get another power while the Metal Cap is in effect, they will combine; this is possible especially on Dire Dire Docks, where a Metal Box is located directly beside a Exclamation Mark Block.
In Super Mario 64 DS, the Metal Cap does not appear. Instead, Wario becomes Metal Wario when he comes in contact with a Power Flower, but still retains the same properties as Metal Mario. As Wario couldn't use other Cap forms, it was not possible to get a Vanish Metal form in this game.