An all-terrain vehicle (ATV), also known as a quad, quad bike, three wheeler, or four wheeler, is defined by the American National Standards Institute (ANSI) as a vehicle that travels on low pressure tires, with a seat that is straddled by the operator, along with handlebars for steering control. As the name implies, it is designed to handle a wider variety of terrain than most other vehicles. Although it is a street-legal vehicle in some countries, it is not street legal within most states and provinces of Australia, the United States, Canada, or the United Kingdom. In the UK, a recent variant class of ATV is now road-legal, but there are few models available in this class.
By the current ANSI definition, ATVs are intended for use by a single operator, although some companies have developed ATVs intended for use by the operator and one passenger. These ATVs are referred to in this article as tandem ATVs.
The rider sits on and operates these vehicles like a motorcycle, but the extra wheels give more stability at slower speeds. Although equipped with three (or typically, four) wheels, six-wheel models exist for specialized applications. Engine sizes of ATVs currently for sale in the United States, (as of 2008 products), range from 49 to 1,000 cc (3 to 61 cu in).
Reference: Wikipedia Quad Biking