Controller

From Yu-Gi-Oh!

The Controller of a card is usually the one who has it on their side of the field. Controller can change the battle position of the monster (unless something targets it). If the monster is face-down then only the controller of the monster can see the card. Your opponent can not see the face-down card unless if the monster is targeted to reveal the monster or they are the new controller. The effect of a card is applied normally if control is changed.

Example:

Player 1 Sets Dharc the Dark Charmer.

Player 2 Summons Destiny Hero - Defender.

Player 1 Flip Summons Dharc, and uses his/her ability to take control of Defender and changes Defender into Defense Position.

Player 2 Draws a Card because of Defender's effect.

Even though Player 2 owns Destiny Hero - Defender, Defender is on Player 1's side of the field. Player 2 is Player 1's opponent, so Player 2 draws the card.

If Defender were to be removed from the field, it would be moved to Player 2's Deck, Graveyard, etc.

Reshef the Dark Being, the Charmers, Brain Control, Change of Heart, Enemy Controller, Mark of the Rose and Snatch Steal are all capable of switching control of monsters.

There are also certain monsters that have effects that prevent change of control or are activated when control is changed.