shocks and springs are an inexpensive alternative to full coilovers to give your car a lower, stiffer, more sporty ride. Lower springs with a stiffer spring rate will make your car ride rough, the tradeoff is reduced weight transfer and greater stabillity when cornering.
Shocks keep the springs on your car from constantly bouncing by providing resistance to the lateral motion of the shock and spring assembly.
Aftermarket shocks usually have features like adjustibility or support for lower height springs.
Springs dictate the ride height of the car and the motion of the dampers.
Stiff, low springs allow of reduced weight transfer when accelerating and cornering at the cost of reduced ride comfort.