Until the late Middle Ages, all boats were steered by a tiller located near the rear of the boat on the right side facing the bow. The tiller was called the steerboard. This side was later renamed the starboard side. This was now called the port side. Small crafts are boats.
Large crafts are vessels.
The difference between a boat and a vessel is a boat can be carried on a vessel, but a vessel cannot be carried on a boat because of its much greater size. The only large vessels that are still called boats are the submarines of the World's Navy.