Trek 4500
5 yrs?
$460 + about $300 in upgrades over time
Blue
Married
Pacific Northwest
...

Please take my noob knowledge for what it's worth. But it's my understanding that brands make a range of forks. RST in the U.S. mostly makes entry level forks for stock parts on bikes, aftermarket RST forks can be entry level or up to mid level, but no great ones. I've heard RST in Europe has some really nice forks that are difficult to get here in the U.S.. Manitou makes entry level forks that are a little better than the entry level forks, and again up to mid level forks, i don't know what Manitou's high end forks are like. Rock Shox makes forks that start out at the higher end of entry level forks, still entry level but much better than RST's entry level forks. Rock Shox's line has entry level, mid level and high end forks. Fox makes forks that start out in the mid range and go up from there. So you could find an RST fork that beats a Rock Shox fork and a Rock Shox fork that beats a Fox fork and so on.

Since the OP asked what the difference is and i don't think anyone has commented on the feel other than "its better"...
My experience with forks is pretty limited but I've done a lot of reading and talking to LBSs to determine what I need for my riding (XC). I am a noob so take my advice for what it's worth, feel free to flame if it makes you feel better. The lower end forks just have a spring in them, the fork is 0% compressed when in the normal riding position, when a bump is hit, the spring compresses to absorb the bump, and then decompresses back to 0%. the "preload" adjusts how much load is already put on the spring, making it firmer, harder to compress, less preload will make it softer (better at absorbing bumps, but if it's too soft it will bottom out easily-no fun). Since the spring has nothing fighting it when it decompresses, it can bounce back pretty hard, making the bike difficult to control or bucking you off, thus the need for rebound control. rebound is another control that keeps the fork from decompressing too fast, you can control it to make it bounce back quickly or slowly. Too quickly and it can still buck you off, too slowly and many successive bumps can work the fork down and it can still bottom out. My aforementioned trek has a rockshox judy with preload but no rebound, it has never bucked me off though. From what I've read motion control is like rebound control but it makes the bike feel very soft and plush but doesn't compress when it doesn't need to, making it a fairly stable pedaling platform too. The benefit of extra travel is the obvious, extra travel. the shock has farther to compress so it can be softer at the top and firmer at the bottom, with more in between. some forks have adjustable travel too, so you can stretch it out and have a 140mm fork for aggressive terrain, then when you get on long smooth singletrack you can set the fork to 80mm when you don't need it. I saw a Fox fork (Talas?) that lets you basically bottom out the fork so the bike is leaning very far forward, making you basically horizontal when going up steep climbs. Forks are also supposed to have a little sag (~10%) this is where the fork compresses a little just with you sitting on it. (my fork doesn't sag at all, many cheap forks don't). fork sag allows the wheel to move down when it hits a depression in the trail, maintaining contact with the ground and increasing traction. a fork's ability to keep the wheel on the ground in depressions and bumps i believe is called "tracking". A lockout is when you can "turn off" a fork, so it doesn't compress at all. most lockouts have a "lockout limit" so if you have your fork locked out and take a jump the fork will compress instead of crushing its hi-tech innards. some forks have "remote lockout" where you get a button or a knob on the handlebar that controls the firmness or lockout function of the fork on the fly. you can also get a remote lockout on some rear shocks too. also, nicer forks tend to be lighter (if you stay in the same class XC-XC AM-AM) which is great but it can make them weaker, and might not be a good idea if you do a lot of downhill or jumps. ok thats all for now, again, most of this is stuff i've read not seen (except the severely adjustable fox fork) so take it all with salt as I could be wrong.
oh some stuff on types of forks, cheap forks (rst, manitou, rock shox) pretty much just have the spring, as you get into more expensive forks (manitou, rockshocks, marz, fox) they can have elastomers, liquid, or air. i think some cheap ones can have elastomers too.