A coil spring is a standard cylindrical spring. In the automotive world coil springs are made from steel, and are commonly used in suspension, (they provide a much smoother ride than a leaf spring ever could). The front of most vehicles will have two coil springs, (mounted to the upper and lower control arms), with a shock in the middle.
Automotive coil springs are under a great deal of pressure and must be compressed to be installed or removed. Attempting to remove such a spring without compressing it can result in injury or even death.
Cars and trucks can be customized for lower, (or higher), ride height by installing shorter or taller coil springs. It is also possible to "cut down" or "heat up" a coil spring to acheive a lowered height. But doing that ruins the spring, and completely changes the suspension geometry of the vehicle, (making it difficult to align).