Control is the term used to describe who can use what card on the field. Most of the time, cards remain under the control of their owners, but a few cards can bend this situation.
Change of Heart, Enemy Controller, Creature Swap, Mind Control, Dark Necrofear, Brain Control, Snatch Steal and Brain Jacker are all capable of taking control of your opponent's monster.
When a monster switches Control to you, place it on your side of the field. It is returned to the original owner's Graveyard when sent to the Graveyard in any way.