Two major differences are chopper gun vs hand laid fiberglass. Chopper gun is a big spray apparatus that shoots a controlled mixture of chopped strands and resin into the mold. Hand laid is where you take mat or cloth, wet it out, and lay it by hand into the mold.
Another difference is polyester resin vs epoxy resin. Most cars are built with polyester (and so are boats) and it is fine for the job. Epoxy is stronger, bites better, and is much more expensive. Not needed for most body building applications, but does work better when you go back to repair a section or add something as it bonds much better.
There are also a variety of fiberglass products like mat, cloth, roving, etc. Generally you use mat as the layup material against your gelcoat because it leaves less of a pattern, then you can use cloth for the next layer if you wish, or not at all.
If you are going to be doing any serious fiberglassing I would suggest you get a good book on boat building as most of the techiques and products are the same.
Don
