A zone is one of several places where a card or object can be placed during a game. 
From the Comprehensive Rules (as of Dark Ascension)
- 400. General
- 400.1. A zone is a place where objects can be during a game. There are normally seven zones: library, hand, battlefield, graveyard, stack, exile, and command. Some older cards also use the ante zone. Each player has his or her own library, hand, and graveyard. The other zones are shared by all players.
- 400.2. Public zones are zones in which all players can see the cards' faces, except for those cards that some rule or effect specifically allow to be face down. Graveyard, battlefield, stack, exile, ante, and command are public zones. Hidden zones are zones in which not all players can be expected to see the cards' faces. Library and hand are hidden zones, even if all the cards in one such zone happen to be revealed.
- 400.3. If an object would go to any library, graveyard, or hand other than its owner's, it goes to its owner's corresponding zone.
- 400.4. Cards with certain card types can't enter certain zones.
- 400.4a If an instant or sorcery card would enter the battlefield, it remains in its previous zone.
- 400.4b If a plane, vanguard, or scheme card would leave the command zone, it remains in the command zone.
- 400.5. The order of objects in a library, in a graveyard, or on the stack can't be changed except when effects or rules allow it. The same is true for objects arranged in face-down piles in other zones. Other objects in other zones can be arranged however their owners wish, although who controls those objects, whether they're tapped or flipped, and what other objects are attached to them must remain clear to all players.
- 400.6. If an object would move from one zone to another, determine what event is moving the object. If the object is moving to a public zone, all players look at it to see if it has any abilities that would affect the move. Then any appropriate replacement effects, whether they come from that object or from elsewhere, are applied to that event. If any effects or rules try to do two or more contradictory or mutually exclusive things to a particular object, that object's controller - or its owner if it has no controller - chooses which effect to apply, and what that effect does. (Note that multiple instances of the same thing may be mutually exclusive; for example, two simultaneous “destroy” effects.) Then the event moves the object.
- 400.7. An object that moves from one zone to another becomes a new object with no memory of, or relation to, its previous existence. There are seven exceptions to this rule:
- 400.7a Effects from spells, activated abilities, and triggered abilities that change the characteristics of a permanent spell on the stack continue to apply to the permanent that spell becomes.
- 400.7b Prevention effects that apply to damage from a permanent spell on the stack continue to apply to damage from the permanent that spell becomes.
- 400.7c If an ability of a permanent requires information about choices made as that permanent was cast as a spell, including what mana was spent to cast that spell, it uses information about the spell that became that permanent as it resolved.
- 400.7d Abilities that trigger when an object moves from one zone to another (for example, “When Rancor is put into a graveyard from the battlefield”) can find the new object that it became in the zone it moved to when the ability triggered, if that zone is a public zone.
- 400.7e Abilities of Auras that trigger when the enchanted permanent leaves the battlefield can find the new object that Aura became in its owner's graveyard if it was put into that graveyard at the same time the enchanted permanent left the battlefield. It can also find the new object that Aura became in its owner's graveyard as a result of being put there as a state-based action for not being attached to a permanent. (See rule 704.5n.)
- 400.7f If an effect grants a nonland card an ability that allows it to be cast, that ability will continue to apply to the new object that card became after it moved to the stack as a result of being cast this way.
- 400.7g A resolving spell or activated ability can perform actions on an object that moved from one zone to another while that spell was being cast or that ability was being activated, if that object moved to a public zone.
- 400.8. If an object in the exile zone is exiled, it doesn't change zones, but it becomes a new object that has just been exiled.
- 400.9. If a face-up object in the command zone is turned face down, it becomes a new object.
- 400.10. An object is outside the game if it isn't in any of the game's zones. Outside the game is not a zone.
- 400.10a Cards in a player's sideboard are outside the game. See rule 100.4.
- 400.10b Some effects bring cards into a game from outside of it. Those cards remain in the game until it ends.
- 400.10c Cards outside the game can't be affected by spells or abilities, except for characteristic-defining abilities printed on them (see rule 604.3) and spells and abilities that allow those cards to be brought into the game.
- 400.11. Some effects instruct a player to do something to a zone (such as "Shuffle your hand into your library"). That action is performed on all cards in that zone. The zone itself is not affected.