The rear wheel arch is the arched metal section which sits above the wheel, arching over it, providing a space in the car body, for the wheel to sit. 

Bolted or fixed with clips to the wing of the car, the wheel arch arches above and follows the contours of the rear wheel. It's usually covered with a plastic wheel arch lining or moulding. 

The wheel arch functions to provide a space which houses the wheel, keeping the wheel within the body of the car, which provides a safety element in a crash by eliminating the risk of the wheels spinning off the car and out into the road and any unsuspecting traffic/pedestrians. Also assisting with the overall aerodynamics of the car, the <Model> wheel arch helps to direct the air flow over the vehicle. The rear wheel arch also functions as a protective barrier, helping to stop mud, stones and other debris from the road from hitting the car.

Exposed to all weather conditions and damage from stones and other road debris, the wheel arch will at some point wear out. This part is particularly susceptible to corrosion and cracking and you'll eventually need to buy a new one. Serious damage can occur if impacted in a crash and a replacement wheel arch will need to be fitted.  Wheel arches come in a variety of designs and are specific to car make and model.  When you buy a good quality replacement wheel arch you can be certain you'll get a part that fits exactly with the make and model of your car.