The typical suspension system features a controlling arm, coil spring and a shock absorber or a strut. The difference between a strut and a normal shock is that a strut carries some of the weight of the car. Most of the time, the spring is mounted on the strut. In case of any compression on the strut, or the spring, or both, the wheel moves upwards towards the body of the car. The mechanism of the coilover is similar. It has a longer coil spring that is coiled around the strut. For the upward movement of the wheel, both the shock and the spring must compress. The spring carries the weight upon compression while the strut transfers or damps the bouncing to the spring.
In the case of coilovers, you get what you pay. That is to say that the more expensive coilovers will achieve the best handling performance for your vehicle. A cheaper coilover may provide the same functionality as an expensive coilover. However, the cheap coilover will not provide the same durability, safety, and performance as a well-designed quality coilover.