The Poop Deck was the aftermost part of the Titanic. The Third Class passengers didn't have a real promenade, but the Poop Deck served as one.
The Docking Bridge was elevated on this deck. Many benches were placed for passenger use. Overhanging the stern was a flagstaff with a flag of the White Star Line. The forward end of the deck was taken up by 2 cargo cranes. The Poop Deck was 128 feet long.
When the ship sank, the Poop Deck was the very final deck to submerge. The passengers already realized that the Poop Deck would be the last place to stay dry, so ca. 1100 people gathered there to await the final moment.
The term "poop" comes from the latin word puppis meaning the aft most deck of a ship. Such deck was originally intended to help guard the rear end of a ship from being swamped by large waves. But now ships were much bigger so a poop deck wasn't necessary anymore. The Titanic was the last ship to have a poop deck, it was not included on the Britannic (which was already under construction before Titanic sank) or any other ship later built.