There are three major materials that kayaks are made from. These materials are Rotationally Molded Plastic (Rotomold), Fiberglass, and Kevlar. Here are descriptions and comparisons of the different materials.
Rotomold - Rotomold kayaks are basically indestructible. You can paddle them over rocks, drag them down trails, and throw them off the roof of your car or truck and the only signs you will see are a few dings and scratches. Rotomold is also the material that is most widely used among kayak manufacturers. This material is also the cheapest in cost of all three major materials, but also weighs about ten percent more than fiberglass does. The heavier weight does not affect how well the kayak floats.
Fiberglass - The benefit of a fiberglass kayak is that it is much lighter than a rotomold kayak, but you can also expect to pay about 15-20 percent more for a fiberglass kayak than you would for a rotomold kayak. You also have to take better care of a fiberglass kayak. You have to pay attention to any rips in the fiberglass, which will need to be patched. You will also have to wash off the kayak after any saltwater exposure so that the salt does not eat away at your kayak. But the durability of a fiberglass kayak is quite satisfactory as long as you are not dragging it across or bouncing it off rocks.
Kevlar - The final major type of kayak materials is Kevlar. The same material that is in bulletproof vests used by military branches and police forces. Kevlar is the lightest material you can get today. It is significantly lighter than fiberglass is, and is incredibly strong. A 19 foot Kevlar kayak is about 21 pounds lighter than a fiberglass one, and as strong as rotomold.