A trapdoor is a block that is used as a horizontal door, it fills a one-block space and allows players to pass through them. They can be opened by right-clicking them or when powered with a redstone signal.
An acacia trapdoor (via screenshot)
A variety of trapdoors generate in naturally generated structures such as villages, ancient cities, shipwrecks, and igloos. Most trapdoors are made from wooden materials and these can be picked up by breaking them with an ax. Iron trapdoors on the other hand require players to use a pickaxe to pick them up.
Wooden and iron trapdoors have different crafting recipes as to make a wooden trapdoor players will need to fill up the bottom three columns of the crafting grid with 6 of the desired type of wood planks. On the other hand, to make an iron trapdoor players will need to use 4 iron ingots and place them in a 2x2 pattern in the crafting grid.
Crafting jungle trapdoors (via screenshot)
Trapdoors can be used as a switchable barrier to entity movement. Although they are primarily used to block movement by mobs and players, a trapdoor can also be used to control the movement of boats, for example, a top trapdoor placed in a two-wide water flow stops a boat when closed but allow it to move again when open. Iron trapdoors can be opened only with redstone power.
A trapdoor at the top of a ladder denying mobs entry to the room above (via screenshot)
Trapdoors are hatch-like structures that are used most commonly as doors. Wooden trapdoors can be opened by simply right-clicking them while iron ones require redstone power. All the wooden variants can also be used as fuel in furnaces but they are quite inefficient since they only smelt 1.5 items.
A. Yes, water will flow around a trapdoor.
A. No, mobs cannot open or even break a trapdoor making them quite useful in harder difficulty modes.
A. If the trapdoor has visible opening light will go through it.