Fiberglass is a "composite material" (two or more dissimilar materials mixed to provide a part with the best characteristics of each) and consists of fibers captured in a plastic resin matrix. The resin is liquid in raw form and catalyzed to harden and hold the fibers into the orientations that give the final parts their strength and stability of shape.
My fabricators use marine-grade polyester resin with “E-glass” non-woven cloth. This is the most cost effective system for automotive applications.
We almost exclusively lay up our fiberglass by hand, as opposed to spray lamination ("chopper gun" or "slobber gun" as one of my customers humorously calls it).
"What's the difference?" Spray lamination is a process whereby long, continuous strands of fiber, or "tow," are fed through a spray gun that chops the strands into two- or three-inch long pieces, wets them with resin and catalyst, and blows them onto the surface of the mold. It is a bit like spray painting.
Chopper gun production can actually result in pretty nice parts. The quality is highly dependent upon the experience of the operator and the care with which he handles the gun. The key is controlling resin-to-fiber ratio and uniformity of thickness (i.e., not holding the gun over some areas longer than others).
Hand lamination starts with dry fabric. It can be woven or non-woven. Woven fabric looks just like cloth but tends to be thinner. Non-woven fabric (aka “chopped strand matt”) looks like spray-laminated fiberglass when cured, but is very consistent in its thickness. The laminator (or "fiberglass guy") cuts the fabric off a roll, wets it with resin (which he has mixed with catalyst in a bucket), and lays it into the mold by hand.
Regardless of how the fiber and resin get into the mold (by gun or by hand), the laminator rolls the material out by hand, forcing out air bubbles and increasing the strength of the final part.
Generally, hand-laminated parts are going to be slightly denser and stronger than a chopped part of equivalent weight (or, conversely they can be made ligher for an equivalent strength). This is more important as you get into thinner parts, like for racing applications.
I strongly recommend hand lamination for all parts, but especially for parts that will be bolted onto your car, for example front fenders (as opposed to rear quarter panels which are bonded on). I also recommend hand lamination for any parts that are installed in such a way that the underside may be seen, for example your hood.
I prefer gray “sanding primer” gel coat, but can do other colors for small additional cost. I think it was $200 extra last time I did a color change for a complete kit, and that's just the cost difference of the gel coat.
Alternative resin systems (vinylester, epoxy) and other fibers are available. However, they add significantly to the cost.
Here's a little philosophy about alternative fibers (Carbon fiber, Kevlar, etc.): If you have a specific need for such materials (maybe you are building a Formula One car or an airplane), you know what you are doing, and you know exactly the benefits of certain materials in certain applications, alternative fibers can be great. However, if you only want them because you think they are fashionable, I'm happy to help you waste your money. I can do it, but it will cost you, usually about double the cost of fiberglass.
Carbon fiber is a lot “stronger” than fiberglass and can be used to make some extraordinarily lightweight parts. However, to fully realize the difference you need to use an advanced resin system and to autoclave (cure under heat and pressure) the part. This will increase the cost by a factor of at least ten.
For a given weight, raw carbon fabric is about twice as strong in tension as E-glass. Theoretically, you could make a part half as heavy. However, the dry fabric is only a portion of the weight of a final part. Even if you could achieve 50/50 resin to fiber ratio (a very good ratio) cutting the weight of the fiber in half will only reduce the weight of the part by 25%.