A three-wheeler is a vehicle with three wheels, either "human or people-powered vehicles" (HPV or PPV or velomobiles) or motorized vehicles in the form of a motorcycle, all-terrain vehicle (ATV) or automobile. Other names for three-wheelers include trikes, tricars and cyclecars. The term tricycle is used somewhat interchangeably, but the term three-wheeler is more often applied to motor vehicles. They can be legally classed as either automobiles or motorcycles.
Many three-wheelers which exist in the form of motorcycle-based machines are often called trikes and often have the front single wheel and mechanics similar to that of a motorcycle and the rear axle similar to that of a car. Often such vehicles are owner-constructed using a portion of a rear-engine, rear-drive Volkswagen Beetle in combination with a motorcycle front end. Other trikes include ATVs that are specially constructed for off road use. Three-wheeled automobiles can have either one wheel at the back and two at the front, (for example: Morgan Motor Company) or one wheel at the front and two at the back (such as the Reliant Robin).
Due to its superior safety when braking, an increasingly popular form is the front-steering "tadpole" or "reverse trike" sometimes with front drive but usually with rear drive. A variant on the 'one at the front' layout was the Scott Sociable, which resembled a four-wheeler with a front wheel missing. Not illustrated is the Thebis rear-steered delta from Sidney, British Columbia
Three-wheeler cars, usually microcars, are often built for economic reasons: in the UK for tax advantages, or in the US to take advantage of lower safety regulations, being classed as motorcycles. As a result of their light construction and potential better streamlining, three-wheeled cars are usually very economical to run.
Three-wheeler transport vehicles known as Auto Rickshaws are a common means of public transportation in many countries in the world. Auto rickshaws are an essential form of urban transport in many developing countries such as India, and a form of novelty transport in many Eastern countries.