Companies like McMillan, Brown Precision, MPI, Manners, and Hi-Tech Specialties use laminated layers of either fiberglass or Kevlar cloth to make up the bulk of the stock. This cloth is laminated together with epoxy from the grip up through where the front swivel is attached. The cut out for the action is done with milling machines exactly the same way wood stocks are made. The outer layers of the forend and buttstock have several thin layers of fiberglass or kevlar laminated to form a shell for the exterior of the stock. The inside of those areas are hollow, but filled with foam to deaden sound.
Kevlar is much lighter, and stiffer than fiberglass. It is also more expensive. Since it has no flex during recoil it CAN be more accurate and lighter. But the stiffness also makes it possible to break a little easier. Not recommended for anything that kicks more than 300 WM.
Cost of these stocks are in the $500-$600 range with Kevlar at, or slightly over $600. Building them is labor intensive with 3-4 month wait times in many cases. All are of similar quality, it comes down to which stock design you like best. McMillan offers by far the most options to fit a wider variety of rifles and in many different stock styles. Most of their stocks can also be taken out of the box and bolted on. Usually with no fitting. Most of the others need to be put on the action by a skilled gunsmith. Most fiberglass stocks will be in the 30-32 oz range. The kevlar stocks 16-24 oz. A typical wood stock is the same as fiberglass at 30-32 oz just for reference.
HS-Precision and Bell & Carlson use an aluminum chassis in place of the laminated cloth the better stock makers use. They do use some layers of cloth to form the outer skin of the stock. But the rest of the stock is melted plastic, with chopped up fiberglass (think attic insulation) poured into a mold. There is no machining. Price is in the $275-$400 range. Most of these will weigh 36-40 oz.
B&C still makes a few with no metal chassis. Those are very poorly constructed and should be avoided. The plastic factory stocks are much tougher. Those with the metal chassis are pretty rugged and can be accurate. But the metal chassis results in a heavier stock that can be chunky and thick through the grip and fore end. I have no problem with them on a rifle intended for target/varmint/tactical shooting where you want a heavy rifle. I don't like them on a general purpose hunting rifle.
To be honest, most of the factory plastic stocks shoot just fine. I currently have 4 rifles sitting in McMillan Edge stocks made with Kevlar. I like them a lot, but none of the rifles shoot any better now than they did in the factory stocks. They are 1/2 lb lighter, more rugged, and do look and feel a lot better. I have some other rifles sitting in cheap plastic stocks that shoot every bit as good. Just don't look as good.