A car moves on the road at a great speed. And though the road surface is pretty smooth it is not perfectly flat. Therefore at the high speed of the car even a minor imperfection should cause the car to get thrown off and the passengers in the car should get rattled. But that does not happen. This is because the car's passenger cabin is mounted on a suspension which has the ability to absorb the imperfection of the road and not transmit the effect of the imperfection to the passenger seats.
In the early days the suspension was done using springs that would compress and expand and give the car passengers a smooth ride. Later with the advent of hydraulics, telescopic hydraulic suspension devices were used which were able to deliver very high levels of smoothness to the ride.
Though the spring type and hydraulic type shock absorbers performed the same function yet they were not exactly the same. And for different levels of imperfections on the road there performance was different. So if one wanted to have the best shock absorber for all levels of imperfection on the road then one needed to install a combination of both of them. This is what coilovers essentially are. It is a hydraulic shock absorber with a coil over it and when there is a perturbation both the hydraulic and the spring components combine to absorb it.
Since a coilover is a combination it doesn't require as long a displacement to absorb the shock. So you car need not be mounted on a long suspension element and your car can have a lower suspension. A lower suspension lowers the center of gravity of your car and makes it more stable. This is particularly useful for high performance cars.
Coilovers are not very expensive and you can easily buy them online. You can read details and compare options from the comfort of your home. You should though take care to have them installed by professionals. You should therefore provide for the time and cost of doing that while planning the purchase of the coilovers. You can see a wide range of coilovers at www.ilovebodykits.com