Door

Doors are blocks that are only obtainable through crafting. Upon clicking a wooden door with either the left or right mouse button the block will shift from its native closed state to an open state allowing passage; clicking the door once more will return it to being a closed door. If two doors are placed adjacent to each other, they will orient with their handles inward, creating a double door.

Destroyng the bloock beneath a door will break it. Swords are also extra-effective against wooden doors.

Iron Doors
An Iron Door acts the same as a wooden door with the exception of only being able to be toggled by a switching mechanism such as a Lever or a Stone Button. If an iron door has both an on and off signal connected at once, the off signal takes precedence. This creates the possibility to make doors that are only controllable from remote locations using a switch and Redstone.

Iron doors can take quite a bit of time to destroy even with a pickaxe. To destroy them even more quickly, simply destroy the block beneath the door.

Glitches
Because the door occupies the edge of a tile, if you stand in the center of the tile you can hit things on the other side. Make sure you place the door from the outside of your house so that you can attack monsters on the other side, but they cannot attack you. As long as you follow this rule and make sure the door is on the edge that will be exposed to danger, monsters cannot harm you from the outside, but you can fight them without opening the door.

Minecarts and boats will not move through doors. Water will not pass through either, leading to an excellent way to keep multiple rooms from flooding. Another practical use of the door's water glitch is to leave a door open as an entrance to something underwater that is water-free on the other side.

Doors may be placed on doors, but it will be lost, and nothing will happen.

Be careful when opening doors with right click while holding a filled bucket. Doing so will empty the contents of the bucket at that location. Since no tool can break a door in one hit, I would recommend opening and closing doors with a melee (left click) to avoid this.

If the bottom block of a door is destroyed, the door is destroyed and not recoverable. This does not occur with the top block. --I personally have never had this happen. It may be a multiplayer-only glitch. More research will be performed shortly.

There is also a small glitch involving double doors and switches that causes one door to be in the opposite state in which it's switch should reflect. In other words, a door with it's switch pressed will stay closed, and when the switch is no longer activated, the door will open

Another glitch can occur, if you use both left and right mouse button to repeatedly click the door, using a small offset between the left and right clicks. This will cause the door to split, as seen on the picture. The door may move by itself some time afterwards, moving back to original position, as well as not.

This picture was taken using the Minecraft server to host a game. This still happens in Alpha.