A wood door
The Door is a block in minecraft that functions as a changing-state block. It is two blocks high and destroying either block will destroy both, giving one door item. Wooden doors, like this one, can be opened and closed (turning nintey degrees on a corner) with either a left click, right click, or redstone. Iron doors, which are tough to break down, can only be opened with redstone current or by powering adjacent blocks. Blocks that will open it are every block directly adjacent to the door itself. Besides the obvious usage as a door, doors have a slightly more obcure usage. By opening and closing, they can act as tightrope-style drawbridges, opening and closing a small path along their top edge, or letting things fall when the edge disappears from under it. Before the hatch was added, they could be used to let items fall into a colletion point or dispence minecarts. However their use is largly deprecated as pistons and hatches fill that place, and they are far less unwieldy, taking less space (1x1x1 or 1x2x1, not 2x2x1).