The wheel is the name of a circular disc or torus shaped object, the fundamental operation of which is to transfer linear motion (that is, going along) into rotary motion (that is, going around). It is one of the simple machines.
Wheels can be fastened on a wheel axle and attached to a vehicle. The wheel will then roll over the ground. Compared to a vehicle without wheels, such as a sled, the friction from "ground against vehicle" is largely eliminated and replaced with friction from "wheel axle against socket", where a lubricant or group of bearings may minimize it. Movement is thereby achieved with much less friction which means much less work is necessary.
With the reduction of friction and the lower amount of work necesary to move items, the wheel is essential in transport. In order for wheeled vehicles to work well, one must employ them on roads as a part of road transport, or fit the vehicle with superior suspension. A development of wheels to allow them to travel through rough terrain is the track found on some tractors and armored vehicles.
Since a wheel is a rigid object, it will only be non-rotating when all the torques on it are balanced. Since forces produce larger torques when they are closer to the axis, a wheel can be used to transform between large and small forces applied by friction with belts or other wheels (ie in gears). Other variations on wheels produce the pulley and the windlass.