The cheap forks just have one spring in one tube and if you are lucky there is an adjustment to control the tension. Better ones have two springs, top of the line ones are like dirt bike forks.
The best ones have springs in both tubes with variable compression rates, ie the spring wires get thicker near the ends or there are two springs in each tube, that can be adjusted for how much effort it takes to compress them. Also they are filled with oil that controls the rebound and compression rate as well, this is adjustable by various means, basically the oil has to be squeezed through holes in the fork tubes in order for them to compress at all.. This can be changed by increasing or decreasing the size of the holes or by changing the viscosity of the oil.
And on it goes but this is a start.