Surfing is a surface water sport where a person (the surfer) moves along the face of a breaking ocean wave (the surf). Surfing also takes place on rivers, riding a standing wave. Two major subdivisions within stand-up surfing are longboarding and shortboarding, reflecting differences in surfboard design including surfboard length, and riding style. In tow-in surfing (most often, but not exclusively, associated with big wave surfing), a motorized water vehicle, such as a personal watercraft tows the surfer into the wave front, helping the surfer match a large wave's higher speed, a speed that is generally, but not exclusively a speed that a self-propelled surfer can not match.