This page is intended to cover double-faced cards in detail. As such, it repeats the basic information from the Innistrad Mechanics Article. If you're just looking for the basics, the Mechanics Article should have you covered.
Tens of billions of Magic cards have been printed, and until now, they all had one thing in common: the back of the card. Well, no longer. For the first time ever, Innistrad brings double-faced cards to Magic.
(Confused? Click the "transform" button!)
This article will walk you through how double-faced cards work. The information presented here isn't exhaustive, and may be expanded upon or superseded by updates to the Comprehensive Rules and Tournament Rules with the release of Innistrad. If you have questions that aren't answered here, chances are very good that they'll be addressed in those documents or in the Innistrad FAQ.
A double-faced card has two faces. It has no regular Magic back. Its front face, which is marked with a sun symbol and has a mana cost, is the default. A double-faced card always enters the battlefield with its front face up. This is true whether it enters the battlefield from the stack as the result of being cast, or from anywhere else, such as your graveyard (due to a card like Zombify, for example).
The back face of a double-faced card is marked with a moon symbol, lacks a mana cost, and has a color indicator—that's the dot on its type line—that tells you what color it is. The two faces of a double-faced card are often the same color, but not always. The back face's characteristics matter only if the card is on the battlefield and its back face is showing. Otherwise, only the front face's characteristics count. (For example, Gatstaf Shepherd's converted mana cost when it's in your deck is 2, not zero.)
Each double-faced card has at least one ability that causes it to transform. To transform a double-faced card, you turn it over so that its other face is showing.
To put a double-faced card into your deck, you have two options: You can put your entire deck in opaque card sleeves, as many players already do, or you can use the checklist card provided in many Innistrad packs.
If you're using sleeves, it's pretty straightforward. You'll want to put the double-faced card in the sleeve with its front face showing, and take it out of the sleeve when it transforms. (Putting it back in is optional, at least until it leaves the battlefield.)
The checklist card has a regular Magic back, and its front looks like this:
To use the checklist card, set your double-faced card aside and make a mark on the checklist card next to the name of the double-faced card it represents. Be sure to use a writing implement that won't be visible through the back of the checklist card, and to mark only one double-faced card name on each checklist card.
Only official checklist cards may be used to stand in for double-faced cards in a deck. If you use a checklist card to represent any of the double-faced cards in your deck, you must use checklist cards to represent all of them.
You'll use the checklist card any time it's important to keep the identity of your card secret—in other words, when it's in your library, in your hand, or exiled face down. You'll switch to the double-faced card when the card is on the stack, on the battlefield, in the graveyard, or exiled face up.
Any time a double-faced card is visible—whether because it's in a public zone, because it's revealed (say, by Telepathy), or because it's being looked at by a player due to an effect (say, Coercion)—the players who can see it can see both faces. Any player who can look at a checklist card in a hidden zone may look at the double-faced card it represents.
After a double-faced card transforms, it's still the same card, so any Auras, counters, or other effects stay right where they are (unless the double-faced card's characteristics have changed such that an Aura can no longer legally enchant it).
The word "transform" applies no matter which side is currently showing. In other words, Gatstaf Shepherd might transform into Gatstaf Howler, then Gatstaf Howler might transform into Gatstaf Shepherd.
Although the physical action is the same, transforming is not the same as being turned face down. This means that a double-faced card that transforms will not, for example, trigger Unblinking Bleb. Double-faced cards are always face up; they are never face down. If an effect (say, Ixidron) tries to turn a double-faced card face down, nothing happens.
If any card or token other than a double-faced card is instructed to transform, nothing happens. It won't be turned face down.
All Innistrad Werewolves have the same transform conditions as Gatstaf Shepherd and Gatstaf Howler. Due to Werewolves entering the battlefield at different times, there might be a mix of front-face-up Werewolves and back-face-up Werewolves on the battlefield at any given time.
Not all double-faced cards are Werewolves, and not all double-faced cards transform in both directions.
If an effect copies a double-faced card, it copies the characteristics of whichever face is currently showing. It doesn't copy any characteristics of the other face.
The only objects that can transform are cards that physically have two faces. If a token or a card with a regular Magic back is instructed to transform, instead nothing will happen. Even if a token or non-double-faced card is a copy of one face of a double-faced card, it can't transform.
If a double-faced card becomes a copy of something else, the copied values will overwrite its characteristics for as long as the copy effect lasts, even if the double-faced card transforms. If a double-faced card that's copying something else is instructed to transform, it will do so, because the physical card has two faces, but its characteristics will still be those of whatever it's copying. This is true even if the object it's copying is one face of a double-faced card.