Steamer is an early term for any vessel power by a steam engine rather than sails.
Early steamers were propelled by large paddlewheels. "Side-wheelers", with the paddlewheels on each side, were more seaworthy, and therefore more common in ocean waters, while "stern-wheelers", with a single large paddlewheel at the back, were more common as riverboats. Later, these were replaced by more efficient screw (or propeller) vessels.