Surfing refers to a person or boat riding down a wave and thereby gathering speed from the downward movement. Most commonly, the term is used for a surface water sport in which the person surfing is carried along the face of a breaking ocean wave (the "surf") standing on a surfboard. Surfboards can also be used on rivers on standing waves. Both is sometimes called stand-up surfing, to distinguish it from bodysurfing, in which the surfer does not stand up on the board and only partly raises his upper body from the board. This article focusses on stand-up surfing.
Two major subdivisions within contemporary stand-up surfing are reflected by the differences in surfboard design and riding style of longboarding and shortboarding. In tow-in surfing (most often, but not exclusively, associated with big wave surfing), a surfer is towed into the wave by a motorized water vehicle, such as a jetski, generally because standard paddling is often ineffective when trying to match a large wave's higher speed.
Depending on wave size and direction and on wind conditions, also sailboats surf, namely on larger waves on open sailing waters. Unlike "surfers," sailors usually do not surf in beach waves, and they usually do not go out in order to surf; instead, rather wave and wind conditions may let their boat surf while during a sailing trip. More recently, the same principle of craft-based surfing has been increasingly used by kayakers, notably in the sport of playboating, which is mostly carried out on rivers .
Surfing-related sports such as paddleboarding and sea kayaking do not require waves, and other derivative sports such as kitesurfing and windsurfing rely primarily on wind for power, yet all of these tools may as well be used to ride waves.