Wooden Door

The Wooden Door is a Placeable object, similar to other furniture, with the added benefit of preventing the passage of enemy NPCs. It can be placed in any space so long as it is exactly three blocks high, with a ceiling and a floor.

Town NPCs will open and close doors as they wander about. Players can do the same by right-clicking on them. Enemy NPCs will not open doors except for Skeletons, Dead Miners, Goblins and Angry Bones. Zombies can also open doors during a Blood Moon, and Goblin Peons can break down doors, as if they were destroyed by a player.

Doors can be opened and closed through Wires. If you put wire on one of doors closed state squares, then wiring will toggle closed/open states upon activation, but if you put wires on the side squares, where door opens, then wiring will only close door.

Tips
by rapidly opening and closing them when submerged in the water.
 * Doors open to the left or right based on which direction you face, rather than the direction you are coming from. For example, if a player stands to the left of a door and faces left, the door will open toward the player - to the left. If the player doesn't move and then turns to the right and opens the door, it will now open to the right.
 * Putting furniture (even the second door) on one side of the door prevents it from being opened in that direction. This is useful to keep houses safe from Zombies during the Blood Moon or Goblins, as they'll always attempt to open the doors inward, while still allowing NPCs to go in and out as they please.
 * Doors can also hold back water.
 * In addition to holding back water, doors are also capable in Terraria 1.1 of removing water
 * Doors will break automatically if exposed to lava.

Trivia

 * Even though it is specifically called a wooden door, it is the only door available. It has been hinted that future updates will include other types of doors.