HOW DOES A FENDER WORKS
Fender is the US English term for the part of a vehicle that frames the wheel well (the fender underside).

A fender, though having the same purpose, has different names in other parts of the world. In Britain, the fender is called the wing (this usually refers only to the panels over the front wheels of a vehicle, due to the fact that the rear fenders are seen as part of the vehicle’s body shape).  In Germany, the fender or wing is known as a Kotflugel.

HOW DOES A FENDER WORKS
Automobile fenders are designed to prevent objects such as stones, sand, and mud from being thrown into the air as a result of the rotating tires. These objects can become lodged in the grooves of high speed rotating tires and get thrown around, resulting in damage to other vehicles and even pedestrians; a fender prevents this from happening. A fender also serves safety purposes, because in the event of a collision, they are manufactured to bend so that your engine is as protected as much as possible. Apart from protection and safety, fenders contribute to the overall appeal and aerodynamics of your vehicle.

BICYCLES AND MOTORCYCLES
For a bicycle or motorcycle, the fender or wing is referred to as the mudguard as it guards other road users and the rider as well, from mud and other objects thrown up by the wheels. There are generic fenders that can be fitted to most bicycle frames, but they are relatively uncommon in the more car-dependent English speaking countries, as bikes are more used for recreation and as a result, are only ridden in good weather conditions.

TRUCKS AND AUTOMOBILES
In the current US Auto Industry, only the panels over the front wheels are called fenders. Rear fenders bolted into a quarter panel, were changed to an enlarged welded-on quarter panel, that fulfills both functions, now referred to as the quarter panel.
The terms have evolved and are determined mostly because of their location. Fenders are at the front. Quarter panels are at the rear, with an exception made for dual rear wheel trucks, where the panel at the rear is called a fender.
The bolted panel which covers the wheel on dual rear wheel pickup trucks is called a fender. A pickup truck with a separate bed but without bolt on fenders has a bedside, which performs the function of a fender. When the side of the bed is welded to the cab, as with the Cadillac Escalade and Chevrolet Avalanche, it is called a quarter panel.
You can also add fender flares, fender trims, or fender skirts, which give the body of your car, truck, or SUV additional protection from mud splashes and road debris.
Cars with narrow bodies, such as the Lotus Seven, Allard, and Caterham Seven, use what are called cycle fenders, because of their resemblance to those used on bicycles. They are attached to the wheel suspension and remain at a fixed distance from the tire, regardless of wheel motion, and can therefore be much closer to the tire than fixed wheel wells. This was popular on early Classic cars, because the fenders were lightweight and allowed for a thin streamlined body. Cycle fenders can still be found on ‘vintage’ cars.