A wheel is a circular component that is intended to rotate on an axle bearing.
Wheels are not necessary for the functioning of in-game vehicles, but they greatly help with speed, stability, and manoeuvrability.
Wheels have a health status, and can be shot at, which will pop them.
Use
Stand next to the vehicle and open it's inventory, slide the wheel into it's respective slot.
Weight
Wheels have a large weight, and it's often impossible to carry more than two at a time.
Loot Spawn
Wheels spawn in hardware stores, garages, warehouses, gas stations, and most technical buildings.