There are four moves that steal an opponents held item they are:
Thief is a Dark type move that does 60 Damage to the opponent and also steals the opponents held item if the user is not holding a held item.
Covet is a Normal type move that basically does the same as Thief as it also deals 60 damage to the opponent and steals the opponents held item if the user is not holding a held item.
Trick is a Psychic type that swaps the users held item for the opponents. This is often used on Pokémon that carry a Choice item so that it locks one of your opponents Pokémon into one move.
Switcharoo is a Dark type move that basically does the same as trick and swaps the users held item for the opponents.
Other moves that stop an opponent from using it's held item include:
Knock Off doesn’t steal an opponent’s held item but it does remove the opponents held item so they can’t use it anymore in battle. It also deals 65 damage to the opponent.
Embargo also doesn’t steal an opponents held item but it does stop an opponent from using it’s held item for 5 turns when selected.
Note: Non of these moves will steal or remove items such as a Plates, Mega Stone or any item from a Pokémon with the Sticky Hold Ability.