A sail is a surface intended to generate thrust by being placed in a wind. Depending on the angle between the wind and the surface, one side of the sail will have a higher air pressure than the other one. This difference will cause the sail to pull towards the area of lower pressure.
Sails are primarily used at sea, on sailing ships as a locomotive system, but have been rendered commercially obsolete by other forms of propulsion, such as the steam engine. For recreation, however, sailing vehicles remain popular.