I'll start with a disclaimer: I come at this primarily from a motorcycling perspective and have only just got my first suspended bicycle. However, from what I've read, the principles are the same and the knowledge is transferable.
Pretension (or Preload) and Spring Stiffness
Preload allows a rider to set the "sag". Sag is essentially a measure of how much suspension travel is used simply by the act of the rider sitting in his/her normal riding position. The usual recommendation is that somewhere between 1/4 and 1/3 of the suspension travel be taken up by sag. If, whilst riding, the suspension regularly "bottoms out" (all the spring travel is used up, usually accompanied by a noticable knock), then increase the preload. If this results in sag being reduced significantly below 1/4 of the total travel, then a stiffer spring is probably in order. If the suspension regularly "tops out" (on hard acceleration, or as the wheel drops into a pothole, or over a drop-off), then reduce preload. If this results in a sag much greater than 1/3, then look for a softer spring. If you regularly get topping out and bottoming out, then, assuming you've not wound the damping down really low, then no combination of spring stiffness or pre-load will completely solve it. Personally, I'd err towards reducing bottoming and just live with the topping out.
Bump (Compression) and Rebound Damping
Controls both how fast the suspension compresses and recovers from being compressed. With no damping the suspension would simply bounce up and down after every compression. This can make the bike feel wallowy. Worse, imagine hitting a bump - which compresses the suspension, which then bounces back up, then down - THEN hitting a another bump. The spring, being further compressed than normal, transmits more force to frame and handlebars, making the ride harsh and uncomfortable. In extremis, the spring may go "coilbound" momentarily, meaning it's run out of travel, effectively becoming solid. By adjusting the damping, it should be possible to achieve a comfortable and responsive ride. To gain an appreciation of damping, back it right off and go for a ride. Assuming that you notice the traits decribed above, gradually increase the damping (starting with rebound damping, because that usually has the greatest effect) until the problems are resolved. You can have too much damping. With too much rebound damping, the suspension recovers too slowly after being compressed. With way too much damping, the spring may still be compressed from a bump when you hit the next bump. This compresses it further, the damping again slows its return, whereupon the next bump compresses it even further. And so on. This is know as "jacking down" or "ratcheting down".
Of course, having said all of that, if your suspension feels okay to you, then that's perfect. Just leave it alone and be happy. Or, if you do fancy a fiddle, make a note of the currect settings so that you can return to them if everything goes pear-shaped.
And, yes, spring in one leg and damping in the other is common.
