This article is from the Magic: The Gathering Rules FAQ, by Patrik Linell (pls@claymore.nu) with numerous contributions by others.
The controller of the card is the player who can use its
abilities, attack with it, and so on. If a card says "you", it's always
talking to its controller. Which player is it? That depends. Usually, the
controller and the owner (see below) of a card are the same player, but
there are many effects that can change that. Generally speaking, if you put
a card into play, and no effects apply that could change its controller,
then you control that card.
 
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