There are different types of springs for each kind of automobile and truck depending on your vehicle's suspension design, but the the most common is the coil spring. A coil spring (also called helical spring) is a kind of torsion spring which can store energy and release it later when needed. It can also absorb shock and maintain the force between two contacting surfaces.
Together with a shock absorber, coil springs are pre-assembled as one coil over unit before installation. While the coil spring compresses and stretches to keep you safe as you drive, the shock absorber keeps you from road noise, bumps, and vibrations that you might encounter while doing so. All in all, a coil over is essential in your vehicle's suspension because it gives you optimal damping, good handling and braking, and a comfortable ride all at the same time.