You now may have a basic understanding of what rebound and compression are. Early works of suspension art were pretty crude in the way that they controlled these two actions, if they did at all! Let's take a closer look at what's going on inside your fork.
Compression is usually controllable by a couple of methods. The simplest way to do this is to increase or decrease your springs preload. Preload is the act of loading the spring, or in other words, putting some force on it while it's in a static state. This has the effect of increasing the spring rate, or in other words- it makes the durn thang stiffer! If you back off your preload setting, you take load off the spring, and therefore you have a softer spring. Keep in mind that a soft spring gets compressed easier than a stiff spring. So, in a very crude sense, a fork set up with a stiffer spring from adding in alot of preload in effect slows down the compression stroke. This is still the way alot of lower end forks work. It's not really an accepted method; however, by any enthusiast, racer, or serious cyclist. Now, you see higher end forks with a dedicated air, oil, or oil/coil type compression circuit that is adjustable either inside the fork, or outside with a knob of some sort.
When your fork wants to snap back into it's "normal" state after absorbing a trail irregularity, it's rebound control comes into play. Older technology relied on a simple oil bath piston setup, much like your screen door has on it to keep it from slamming shut. Not adjustable, and usually, not reliable. Today things like oil, air, coil springs, and a combination of all of these are employed in a way that is adjustable to your liking. You can set it up with little rebound control, which will allow your fork to snap back quickly, or add in alot of rebound control for a slower reaction. Usually, this is adjustable outside of the fork by the means of an allen screw, or a knob of some sort.
Obviously, with all of this adjustability, and complexity, todays forks are spendy if you want the luxury of tuning it to your liking. Speaking of tuning, we'll look at that tomorrow. OUT!