Schedule 40 and Schedule 80 refer to the thickness of the walls of pipe.  Schedule is the standard of wall thickness that has been adopted by the American National Standards Institute.

Pipes with Schedule 40 come with standard weight and pipes with Schedule 80 come with extra weight.  The materials used for making Schedule 40 and Schedule 80 come from the same material.

Schedule 80 pipes have a thicker wall than Schedule 40.  As a result, Schedule 80 pipes are stronger than Schedule 40 pipes.  Both pipes are used in construction work, with Schedule 40 being used the most often.  If a strong pipe is needed, consider using a Schedule 80 pipe.