Rudiments, in essence are just simple sticking patterns. The most used one is probably the single stroke roll, allthough many don't consider it a "true" rudiment. Paradiddles are probably the one that's being played the most. Just in case: it's sticking is R L R R L R L L . Now, this sticking can be used in various ways, so in different modulation (8th's, 16th's, quintuplets, triplets, etc). There are 40 rudiments, varying in difficulty. Some incorporate flams, drags and double stroke rolls.
You can get the 40 rudiments at
http://www.drummingweb.com/rudiments2.htm Also check out
http://www.vicfirth.com/education/rudiments.html
Marching drummers are masters at this stuff, but set drummers mostly use it to get some more control (check out the book Stick Control for the Snare Drummer) or to come up with new patterns. Guys like Thomas Lang, Marco Minneman or Terry Bozzio use these to create ostinato's, ie. playing for example a paradiddle on the feet, and soloing on top of it. Offcourse these guys can do the wackiest things with rudiments.
I myself practice them on a pad mostly. I also play them with my feet (allthough I have a little technique problem on my right foot, so I have to correct that first) and
try to play them as ostinato's. My teacher also likes to split them up over the kit, but I find that a little hard to incorporate in your regular playing.
And YES, they do help your playing, alot! It can be quite boring to play them for hours and hours, but it gives you alot more control (especially the day after), and it breaks you away from the single stroke roll that you would normally do.
Necrophagist RULE! So do Malevolent actually! I hope I can find someone to go there, because that's one hell of a show!