Door

A Door is a block obtainable through crafting and found in NPC Villages and Strongholds. Doors come in two varieties: wooden doors made from planks, and iron doors made from iron ingots. Doors are hinged at a corner and have two states: rotated clockwise and rotated counter-clockwise. Villagers are the only mob that can open doors without any redstone-based help, although zombies can destroy wooden doors when the difficulty is set to hard.

The sound of opening and closing of a door can be heard up to 16 blocks away, like mob sounds with the exception of the Ghasts.

Fluids cannot flow through doors, open or closed.

Natural occurrence
Wooden doors are generated on buildings in NPC villages, while iron doors generate as part of strongholds.

Wooden Doors
Using the right mouse button on a Wooden Door will shift it from one state to another allowing or preventing passage. They may also be opened by using levers, buttons, pressure plates, or redstone.

Since zombies will only pound on or attempt to break down a Wooden Door when it is shut, any door placed so that its open state prevents passage will prevent zombies from being able to break down the door. Zombies will wait in front of the door instead. This method is useful for protecting villages or the home of the player. Note that this will also prevent your dogs and cats from pathfinding through the door, so you will need to lead them in or out with leads, or convince them to teleport. It will also make use of redstone or switches far more difficult, as pressure plates and the like will leave the door "closed" (that is, ajar) behind you.

A wooden door can also be protected more obviously, but without the above disadvantages, by putting a fence gate in front (outside) of it.

Iron Doors
An Iron Door is only able to be toggled by a switching mechanism. Iron doors can take quite a bit of time to destroy, even with a pickaxe. To destroy them more quickly, simply break the block beneath the door. Builders can discourage this by placing lava at the top of a 1x1 shaft above the door, and/or by placing the door on top of an obsidian block (or bedrock, if you are in creative mode).

Since villagers cannot operate buttons or levers and zombies cannot break them, iron doors can be used to lock a house in a generated village.

Door Orientation
If two doors are placed adjacent to each other, they will orient with their handles inward, creating a double door. Double doors can be opened by simply placing two pressure plates directly in front of the doors and stepping on them. Each pressure plate will open its respective door, and activating both pressure plates will open both doors.

Boats can also pass through double doors, with the doors acting as gates on canals or docks.

Doors will automatically flip when there is a wall on only the handle side of the door, so that the hinges are on the correct side. However, their default position has the hinges on the left side.[when looking from the face of the door on the border of the block space]

A door does not take up the space of a whole block, so the door is positioned in a certain part of the block space. There is a side on the border of the block, and a side that is about four fifths of a block in. When placing a door, the border side will face you, so if you want to be able to hit monsters through the door of your house, place the door[s] from outside, rather than inside. Doors will always open toward the other side of the block space they occupy, so as not to enter a different block's area.

Trivia

 * In Pocket Edition, standing in the center of a door will mean that you can punch blocks on the other side of the door without any interference.
 * In Pocket Edition, breaking the bottom of a door results in two doors, and the top none, thus allowing one to duplicate their doors infinitely by constantly placing and destroying them (This is no longer the case in 0.7.0).
 * Despite being made of wood, Wooden Doors do not catch fire.
 * Doors placed underwater can be used as air pockets.
 * Block updates next to an opened wooden door will cause it to close.
 * Minecarts can drop through doors if the door is in the right state (open/closed). This is an effective way to control them.
 * If you stand in an open door, then close it, you don't collide until you walk out and back in.
 * While aiming at one half of the door while opening or closing it, if you are positioned in such a way that you are aiming at the opposite half of the door after the door has changed its state, it will open and close until you release the hit/use-key.
 * It is impossible to place doors on an ice block.
 * In SMP, pressure plate lag may cause the server to drag you back behind a closed door when you thought you had already exited.
 * If you manage to be in the door by closing/opening it a few times while being really close to the door can make you jump on the bottom side of it, and if you jump again, being on top of the side door.
 * Placing the door inside out in your home will mean that monsters will be able to attack you through it. You should place the door safely by going outside of your home, and then placing the door in the doorway that you have made. (See the beginner's tutorial)
 * If one harvests the back side of a door, they will notice the "crack" texture being mirrored.
 * If you punch a door in the Xbox 360 Edition it would open.
 * There are unused sounds of Zombies hitting Iron Doors in the game files. These sound files are found in exactly the same directory of those that are of Zombies hitting Wooden Doors.
 * Placing a Wooden Door sideways will prevent zombies from breaking the door down in hard difficulty. If you place a door sideways, it will be functionally open when it's actually closed and functionally closed when it's actually open. The zombies will be tricked into thinking the door is open so it won't try to break the door down and will try to go through but the "closed" door (which in reality is open) will be blocking the way.