Some objects become or turn another object into a "copy" of a spell, permanent, or card. When creatures are involved this mechanic is called cloning. This was first featured on the cards Clone and Vesuvan Doppelganger from Alpha.   
From the Comprehensive Rules (Fate Reforged)
- 706. Copying Objects
- 706.1. Some objects become or turn another object into a "copy" of a spell, permanent, or card. Some effects put a token onto the battlefield that’s a copy of another object. (Certain older cards were printed with the phrase "search for a copy." This section doesn’t cover those cards, which have received new text in the Oracle card reference.)
- 706.2. When copying an object, the copy acquires the copiable values of the original object’s characteristics and, for an object on the stack, choices made when casting or activating it (mode, targets, the value of X, whether it was kicked, how it will affect multiple targets, and so on). The "copiable values" are the values derived from the text printed on the object (that text being name, mana cost, color indicator, card type, subtype, supertype, rules text, power, toughness, and/or loyalty), as modified by other copy effects, by its face-down status, and by “as . . . enters the battlefield” and "as . . . is turned face up" abilities that set power and toughness (and may also set additional characteristics). Other effects (including type-changing and text-changing effects), status, and counters are not copied.
Example: Chimeric Staff is an artifact that reads ": Chimeric Staff becomes an X/X artifact creature until end of turn." Clone is a creature that reads, "You may have Clone enter the battlefield as a copy of any creature on the battlefield." After a Staff has become a 5/5 artifact creature, a Clone enters the battlefield as a copy of it. The Clone is an artifact, not a 5/5 artifact creature. (The copy has the Staff’s ability, however, and will become a creature if that ability is activated.)
Example: Clone enters the battlefield as a copy of a face-down Grinning Demon (a creature with morph ). The Clone is a colorless 2/2 creature with no name, no types, no abilities, and no mana cost. It will still be face up. Its controller can’t pay to turn it face up.
- 706.2a A copy acquires the color of the object it’s copying because that value is derived from its mana cost or color indicator. A copy acquires the abilities of the object it’s copying because those values are derived from its rules text. A copy doesn’t wind up with two values of each ability (that is, it doesn’t copy the object’s abilities and its rules text, then have that rules text define a new set of abilities).
- 706.3. The copy’s copiable values become the copied information, as modified by the copy’s status (see rule 110.6). Objects that copy the object will use the new copiable values.
Example: Vesuvan Doppelganger reads, "You may have Vesuvan Doppelganger enter the battlefield as a copy of any creature on the battlefield except it doesn’t copy that creature’s color and it gains ‘At the beginning of your upkeep, you may have this creature become a copy of target creature except it doesn’t copy that creature’s color. If you do, this creature gains this ability.’" A Vesuvan Doppelganger enters the battlefield as a copy of Runeclaw Bear (a 2/2 green Bear creature with no abilities). Then a Clone enters the battlefield as a copy of the Doppelganger. The Clone is a 2/2 blue Bear named Runeclaw Bear that has the Doppelganger’s upkeep-triggered ability.
Example: Tomoya the Revealer (a flipped flip card) becomes a copy of Nezumi Shortfang (an unflipped flip card). Tomoya’s characteristics become the characteristics of Stabwhisker the Odious, which is the flipped version of Nezumi Shortfang.
Example: A face-down Grinning Demon (a creature with morph) becomes a copy of a face-up Branchsnap Lorian (a 4/1 green creature with trample and morph ). The Demon’s characteristics become the characteristics of Branchsnap Lorian. However, since the creature is face down, it remains a 2/2 colorless creature with no name, types, or abilities, and no mana cost. It can be turned face up for . If it’s turned face up, it will have the characteristics of Branchsnap Lorian.
Example: A face-down Grinning Demon (a creature with morph) becomes a copy of Wandering Ones (a 1/1 blue Spirit creature that doesn’t have morph). It will be a face-down Wandering Ones. It remains a 2/2 colorless creature with no name, types, or abilities, and no mana cost. Its controller can’t turn it face up as a special action. If an effect turns it face up, it will have the characteristics of Wandering Ones.
- 706.4. Some effects cause a permanent that’s copying a permanent to copy a different object while remaining on the battlefield. The change doesn’t trigger enters-the-battlefield or leaves-the-battlefield abilities. This also doesn’t change any noncopy effects presently affecting the permanent.
Example: Unstable Shapeshifter reads, "Whenever a creature enters the battlefield, Unstable Shapeshifter becomes a copy of that creature and gains this ability." It’s affected by Giant Growth, which reads "Target creature gets +3/+3 until end of turn." If a creature enters the battlefield later this turn, Unstable Shapeshifter will become a copy of that creature, but it will still get +3/+3 from the Giant Growth.
- 706.5. An object that enters the battlefield "as a copy" or "that’s a copy" of another object becomes a copy as it enters the battlefield. It doesn’t enter the battlefield, and then become a copy of that permanent. If the text that’s being copied includes any abilities that replace the enters-the-battlefield event (such as "enters the battlefield with" or "as [this] enters the battlefield" abilities), those abilities will take effect. Also, any enters-the-battlefield triggered abilities of the copy will have a chance to trigger.
Example: Skyshroud Behemoth reads, "Fading 2 (This creature enters the battlefield with two fade counters on it. At the beginning of your upkeep, remove a fade counter from it. If you can’t, sacrifice it.)" and "Skyshroud Behemoth enters the battlefield tapped." A Clone that enters the battlefield as a copy of a Skyshroud Behemoth will also enter the battlefield tapped with two fade counters on it.
Example: Wall of Omens reads, "When Wall of Omens enters the battlefield, draw a card." A Clone enters the battlefield as a copy of Wall of Omens. The Clone has the Wall’s enters-the-battlefield triggered ability, so the Clone’s controller draws a card.
- 706.6. When copying a permanent, any choices that have been made for that permanent aren’t copied. Instead, if an object enters the battlefield as a copy of another permanent, the object’s controller will get to make any "as [this] enters the battlefield" choices for it.
Example: A Clone enters the battlefield as a copy of Adaptive Automaton. Adaptive Automaton reads, in part, "As Adaptive Automaton enters the battlefield, choose a creature type." The Clone won’t copy the creature type choice of the Automaton; rather, the controller of the Clone will get to make a new choice.
- 706.7. If a pair of linked abilities are copied, those abilities will be similarly linked to one another on the object that copied them. One ability refers only to actions that were taken or objects that were affected by the other. They can’t be linked to any other ability, regardless of what other abilities the copy may currently have or may have had in the past. See rule 607, "Linked Abilities."
- 706.7a If an ability causes a player to "choose a [value]" or "name a card," and a second, linked ability refers to that choice, the second ability is the only ability that can refer to that choice. An object doesn't "remember" that choice and use it for other abilities it may copy later. If an object copies an ability that refers to a choice, but either (a) doesn’t copy that ability’s linked ability or (b) does copy the linked ability but no choice is made for it, then the choice is considered to be "undefined." If an ability refers to an undefined choice, that part of the ability won’t do anything.
Example: Voice of All enters the battlefield and Unstable Shapeshifter copies it. Voice of All reads, in part, "As Voice of All enters the battlefield, choose a color." and "Voice of All has protection from the chosen color." Unstable Shapeshifter never had a chance for a color to be chosen for it, because it didn’t enter the battlefield as a Voice of All card, so the protection ability doesn’t protect it from anything at all.
Example: A Vesuvan Doppelganger enters the battlefield as a copy of Voice of All, and the Doppelganger’s controller chooses blue. Later, the Doppelganger copies Quirion Elves, which has the ability, ": Add one mana of the chosen color to your mana pool." Even though a color was chosen for the Doppelganger, it wasn’t chosen for the ability linked to the mana ability copied from the Elves. If that mana ability of the Doppelganger is activated, it will not produce mana.
- 706.8. When copying a double-faced permanent, only the copiable values of the face that’s currently up are copied. (See rule 711, "Double-Faced Cards.")
- 706.9. Copy effects may include modifications or exceptions to the copying process.
- 706.9a Some copy effects cause the copy to gain an ability as part of the copying process. This ability becomes part of the copiable values for the copy, along with any other abilities that were copied.
Example: Quirion Elves enters the battlefield and an Unstable Shapeshifter copies it. The copiable values of the Shapeshifter now match those of the Elves, except that the Shapeshifter also has the ability "Whenever a creature enters the battlefield, Unstable Shapeshifter becomes a copy of that creature and gains this ability." Then a Clone enters the battlefield as a copy of the Unstable Shapeshifter. The Clone copies the new copiable values of the Shapeshifter, including the ability that the Shapeshifter gave itself when it copied the Elves.
- 706.9b Some copy effects specifically state that they don’t copy certain characteristics and instead retain their original values. These effects use the phrase "except its [characteristic] is still [value]" or "except it’s still [value(s)]." They may also simply state that certain characteristics are not copied.
- 706.9c Some copy effects modify a characteristic as part of the copying process. The final value(s) for that characteristic becomes part of the copiable values for the copy.
Example: Copy Artifact is an enchantment that reads, "You may have Copy Artifact enter the battlefield as a copy of any artifact on the battlefield, except it’s an enchantment in addition to its other types." It enters the battlefield as a copy of Juggernaut. The copiable values of the Copy Artifact now match those of Juggernaut with one modification: its types are now artifact, creature, and enchantment.
- 706.9d When applying a copy effect that doesn’t copy a certain characteristic, retains an original value for a certain characteristic, or modifies the final value of a certain characteristic, any characteristic-defining ability (see rule 604.3) of the object being copied that defines that characteristic is not copied. If that characteristic is color, any color indicator (see rule 204) of that object is also not copied.
Example: Quicksilver Gargantuan is a creature that reads, "You may have Quicksilver Gargantuan enter the battlefield as a copy of any creature on the battlefield, except it’s still 7/7." Quicksilver Gargantuan enters the battlefield as a copy of Tarmogoyf, which has a characteristic-defining ability that defines its power and toughness. Quicksilver Gargantuan does not have that ability. It will be 7/7.
- 706.10. To copy a spell or activated ability means to put a copy of it onto the stack; a copy of a spell isn’t cast and a copy of an activated ability isn’t activated. A copy of a spell or ability copies both the characteristics of the spell or ability and all decisions made for it, including modes, targets, the value of X, and additional or alternative costs. (See rule 601, "Casting Spells.") Choices that are normally made on resolution are not copied. If an effect of the copy refers to objects used to pay its costs, it uses the objects used to pay the costs of the original spell or ability. A copy of a spell is owned by the player under whose control it was put on the stack. A copy of a spell or ability is controlled by the player under whose control it was put on the stack. A copy of a spell is itself a spell, even though it has no spell card associated with it. A copy of an ability is itself an ability.
Example: A player casts Fork, targeting an Emerald Charm. Fork reads, "Copy target instant or sorcery spell, except that the copy is red. You may choose new targets for the copy." Emerald Charm is a green instant that reads, "Choose one — Untap target permanent; or destroy target non-Aura enchantment; or target creature loses flying until end of turn." When the Fork resolves, it puts a copy of the Emerald Charm on the stack except the copy is red, not green. The copy has the same mode that was chosen for the original Emerald Charm. It does not necessarily have the same target, but only because Fork allows choosing of new targets.
Example: Fling is an instant that reads, "As an additional cost to cast Fling, sacrifice a creature" and "Fling deals damage equal to the sacrificed creature’s power to target creature or player." When determining how much damage a copy of Fling deals, it checks the power of the creature sacrificed to pay for the original Fling.
- 706.10a If a copy of a spell is in a zone other than the stack, it ceases to exist. If a copy of a card is in any zone other than the stack or the battlefield, it ceases to exist. These are state-based actions. See rule 704.
- 706.10b A copy of an ability has the same source as the original ability. If the ability refers to its source by name, the copy refers to that same object and not to any other object with the same name. The copy is considered to be the same ability by effects that count how many times that ability has resolved during the turn.
- 706.10c Some effects copy a spell or ability and state that its controller may choose new targets for the copy. The player may leave any number of the targets unchanged, even if those targets would be illegal. If the player chooses to change some or all of the targets, the new targets must be legal. Once the player has decided what the copy’s targets will be, the copy is put onto the stack with those targets.
- 706.11. If an effect refers to a permanent by name, the effect still tracks that permanent even if it changes names or becomes a copy of something else.
Example: An Unstable Shapeshifter copies a Crazed Armodon. Crazed Armodon reads, ": Crazed Armodon gets +3/+0 and gains trample until end of turn. Destroy Crazed Armodon at the beginning of the next end step. Activate this ability only once each turn." If this ability of the Shapeshifter is activated, the Shapeshifter will be destroyed at the beginning of the next end step, even if it’s no longer a copy of Crazed Armodon at that time.
- 706.12. An effect that instructs a player to cast a copy of an object (and not just copy a spell) follows the rules for casting spells, except that the copy is created in the same zone the object is in and then cast while another spell or ability is resolving. Casting a copy of an object follows steps 601.2a–g of rule 601, "Casting Spells," and then the copy becomes cast. Once cast, the copy is a spell on the stack, and just like any other spell it can resolve or be countered.
- ↑ Sam Stoddard. (January 17, 2014.) "Mirror, Mirror", Daily MTG, magicthegathering.com, Wizards of the Coast.
- ↑ Magic Arcana. (October 23, 2002.) "Clone art", Daily MTG, magicthegathering.com, Wizards of the Coast.
- ↑ Monty Ashley. (May 20, 2010.) "Back to Back", Daily MTG, magicthegathering.com, Wizards of the Coast.