Door

Doors are blocks that are only obtainable through crafting. Upon clicking the object 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 wooden doors are placed adjacent to each other, they will orient with their handles inward.

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. To destroy them 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 will not move through doors. Water won't 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.

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, and won't occur in Alpha, where it seems to have been fixed.