A barque, barc, or bark, is a type of sailing vessel with three masts. They were not very commonly used for merchant ships in the 18th century, and even less commonly as naval warships.
Barques, although uncommon in the 18th century, were very maneuverable vessels. Their foremast and mainmast are square rigged, sometimes carrying staysails, while the mizzenmast has only one very large gaff sail, and a gaff topsail. This kind of rig is not anything special when the ship is sailing with the wind, but does a bit better when its at beam-reach (wind coming directly from one of the sides of the ship). The large gaff sails on the mizzenmast allow for very quick turning when unfurled.