In modern parlance, a ship is a large boat. Ships are some times called boats, but boats are not called ships. The rule of thumb is that a ship is a boat that can carry another boat or boats.
Having said that, as a former crew member on a square rigged ship, I feel compelled to offer the arcane, traditional, age of sail definition: A ship is a sailing vessel with more than one mast and square rigged sails on all masts. A ship may have fore and aft sails as well, but must carry yards (Poles that hang crosswise on the mast from which the sails are hung) on all masts.
I was married aboard the Falls of Clyde, a four masted, full rigged ship.
