The turning radius or turning circle of a vehicle is the radius of the smallest circular turn (ie. U-turn) that the vehicle is capable of making. It is often used as a generalized term rather than a numerical figure. For example, a vehicle with a very small turning radius may be described as having a "tight turning radius".
Two different measurements can be quoted for a vehicle. A kerb turning circle will show the distance travelled by the wheels. The wall turning circle will include an allowance for the width of the whole car, including the overhang of the bodywork. For example, a van may have be quoted as having a turning circle (in metres) of 12.1(K)/12.4(W).
It may be easier to imagine that on a road with low kerbs, you don't need to consider anything other than the tyres (12.1 metres), but if you were moving the vehicle inside a building, the corners of the vehicle might hit the walls so you need to allow for more space (12.4 metres).
A notable exception in this description is of vehicles that are capable of spinning around their central axis, such as a tank or certain lawnmowers as they do not form a circular path as they turn. In this case the vehicle is referred to as a "zero turning radius" vehicle, although whether or not the turning radius is actually nonexistent is unclear.
Some camera dollys used in the film industry have a "round" mode which allows them to spin around their z axis by allowing synchronized inverse rotation of their front and rear wheel sets, effectively giving them "zero" turning radius.