Pre-pregs are reinforcement materials that have been pre-impregnated with either a thermoplastic or thermoset resin. Pre-preg allows you to impregnate the fibers on a flat workable surface, or rather in an industrial process, and then later form the impregnated fibers to a shape which could prove to be problematic for the hot injection process. The resin impregnation process precisely controls the fiber to resin ratio and ply thickness.
Thermoset: Thermoset prepregs are produced by saturating a fiber reinforcement with a liquid thermoset resin. Excess resin is removed from the reinforcement and the resin undergoes a partial curing, changing from a liquid to a pliable solid state.
Thermoplastic: Thermoplastic prepregs are produced by coating fiber reinforcement with a thermoplastic matrix. An advantage to thermoplastic prepregs is the ability to reheat and reform the material multiple times by heating above the melting point of the specific thermoplastic matrix. Unlike thermoset prepregs, thermoplastic prepregs can be stored at room temperature.