There are different decks of a ship, but the primary deck of the vessel is called the main deck. The main deck may be in different locations, depending on the type of ship. It is not usually the topmost deck.
Other decks you might find on a ship include the upper deck, lower deck, promenade deck, flush deck, weather deck, bridge deck, quarter deck and poop deck. The upper deck is on top of the hull of the ship, while the lower deck is either the same as the main deck, or just beneath it, depending on the type of ship. If the ship has a leisure area, this is called the promenade deck. The flush deck is near the front of the ship, the bridge deck contains navigational elements, and the weather deck is a deck that does not contain a roof. The quarter deck is located near the chief mast, and the poop deck is on the ship's stern.