"Schedule 40" and "Schedule 80" refer to reference charts published by the American National Standards Institute (ANSI). When speaking about PVC pipe, the different schedules refer to the size, thickness and maximum pressure of the pipe.
Schedule 40 PVC
Schedule 40 PVC is pipe made of polyvinyl chloride (PVC) that conforms to a set of standards produced by ANSI.
Schedule 80 PVC
Schedule 80 PVC is similar to Schedule 40 PVC, but it is thicker and can withstand a higher PSI.
While schedules 40 and 80 are very common, there are many other schedules for different applications. A Schedule 80 PVC pipe can withstand a higher pressure than a Schedule 40 PVC pipe but not as much pressure as a Schedule 120 PVC pipe.
Different Schedules Used
Smaller schedule pipe tends to be less expensive than larger schedule pipe. Depending on the volume and pressure of material that needs to be moved with the pipe, the different schedules offer a choice in the cost of materials used. If a low-pressure drainage line is being installed, a Schedule 40 PVC pipe would be adequate and more cost effective than a Schedule 120 PVC pipe.
Use caution when working highly pressurized PVC pipes of any schedule. Pressurized PVC can splinter and explode if pressurized beyond the maximum pressure for the schedule and diameter.