In naval architecture, a poop deck is a deck that constitutes the roof of a cabin built in the aft (rear) part of the superstructure of a ship. The fantail is an overhang at the extreme rear of the ship, aft of the poop deck and closer to level with the main deck. The name originates from the French word (la poupe) for stern. This makes the poop deck technically called a stern deck, which in sailing ships was usually elevated as the roof of the stern (or 'after') cabin. In sailing ships, with the helmsman at the stern, an elevated position was ideal for both navigation and observation of the crew and sails.
The picture shows the stern of the ship. The deck out of view above the after cabin is the poop deck. On modern, motorized warships, the ship functions which were once carried out on the poop deck are moved to the superstructure in the center of the ship (or the island on the starboard side, in the case of aircraft carriers).
In nautical parlance, "to be pooped" means to have a wave come over the stern from abaft.