Correctly called Dampers (Velocity- sensitive Hydraulic damping devices). They dampen the vertical motion induced by driving your car along a rough surface. If your car only had springs, it would boat and wallow along the road until you got physically sick and had to get out. Or at least until it fell apart.
Shock absorbers perform two functions. Firstly, they absorb any larger-than-average bumps in the road so that the shock isn’t transmitted to the car chassis. Secondly, they keep the suspension at as full a travel as possible for the given road conditions. Shock absorbers keep your wheels planted on the road. Without them, your car would be a traveling deathtrap.
As you move faster the damper provides more resistance there is to that movement. They work in conjunction with the springs. The spring allows movement of the wheel to allow the energy in the road shock to be transformed into kinetic energy of the un-sprung mass, whereupon it is dissipated by the damper. The damper does this by forcing gas or oil through a constriction valve (a small hole). Adjustable shock absorbers allow you to change the size of this constriction, and thus control the rate of damping.
A modern coil-over-oil unit
The image left shows a typical modern coil-over-oil unit. This is an all-in-one system that carries both the spring and the shock absorber. The type illustrated here is more likely to be an aftermarket item – it’s unlikely you’d get this level of adjustment on your regular passenger car. The adjustable spring plate can be used to make the springs stiffer and looser, whilst the adjustable damping valve can be used to adjust the rebound damping of the shock absorber. More sophisticated units have adjustable compression damping as well as a remote reservoir. Whilst you don’t typically get this level of engineering on car suspension, most motorbikes do have preload, rebound and spring tension adjustment.