Scuds look like a shrimp. Cressbugs (also called sowbugs) are wide and thin. Check out this website for actual pictures.
http://www.troutnut.com/
Both scuds and cressbugs live in weeds. Sometimes you will see a "resourceful" trout swimming/shaking inside a clump of weeds (or even grabbing and pulling the weeds with his mouth) and then swimming downstream and feasting on the scuds, sowbugs and nymphs it dislodged.
Cressbugs are also a bit larger than a scud. Scuds generally are in the 16-20 range (but some are 12-14) and cresbugs are generally in the 12-16 range.
Color and size is important when imitating. Scuds actually turn a lighter color (pinkish to light tangerine/amber) when they molt their shell and turn a darker pink/orange when they are about to die. The easiest way to determine the right size and color is to pull up some weeds at your favorite stream and see what you find. Go from there.
Also, scuds can swim (they dart through the water), while cresbugs, once dislodged, tend to drift freely in the current until they can grab onto something again.
You can tie scuds either using a standard 2x nymph hook and add movement by quick little twitches (swimming and darting) or you can tie them on curved hooks and fish them dead drift.