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Deals damage and steals the target's item, unless they have the abilities Sticky Hold or Multitype, or is Giratina-O. In these cases it will only do damage with no added effect. If the user obtains a Life Orb, it takes recoil damage.
Even though taking the opponent's item is indeed a great thing, Covet is outclassed by other moves for the simple fact that it requires the user to not hold any item itself for it to work. Which is a pretty big deal, especially if that item would give you an edge in a battle. For this fact alone, it's outclassed by Knock Off, which works a bit differently and has less power, but comes with the option of keeping your own held item. Trick is a still more common choice than both of these attacks, as it can combine the merits of retaining a held item and crippling the opponent.
Thief is a Dark-type equivalent.
Knock Off is a weaker attack that removes the opponent's item from the battle entirely.
Trick is a non-damaging move that switches the items of the user and the target with each other.