Composite materials consist of a resin matrix, a reinforcement and sometimes a core. The two main categories of resin are “thermoset” and “thermoplastic.”
Thermoset resins are a one-way street. The chemical curing is a reaction that cannot be reversed. Common thermosets include polyester, epoxy, and vinylester resins. They typically require mixing multiple components to initiate the reaction, and the curing reaction creates heat (is exothermic) and often a volatile byproduct.
Thermoplastic resins are a consistent one-part chemistry that softens when hot and hardens when cool. There is no fundamental change to the material, and thermoplastic resins can be heated and re-formed over and over. Think injection-molding materials, but with long fiber reinforcements. The big benefits are short cycle time for high volume production and potential recyclability. Typical thermoplastic resins used in long-fiber composite applications include ABS, Nylon, PEI and PEEK.
Here is a great video by Fantastic Plastics! on Thermosets and Thermoplastics.