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.
A player implementing a Control based strategy is often going to play such cards as Dark Bribe, Solemn Judgment, Solemn Warning, Seven Tools of the Bandit, Trap Jammer, Trap Stun, Magic Jammer, and various other cards that would give the player a feeling of "control" since these cards all have to do with whether or not the player is going to allow things to pass. The controller of such cards basically says what goes in the duel.
Cards such as "Change of Heart", "Creature Swap" and "Enemy Controller" allow one to take control of monsters owned by one's opponent.
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.
NOTEABLE CONTROL CARDS
Solemn Judgment Solemn Warning Dark Bribe Seven Tools of the Bandit Magic Jammer Magic Drain Trap Stun Trap Jammer