SDR: Standard Dimension Ratio is a standard which ensures the same pressure rating for all diameters of pipe. Whether 1" or 24" in size, pipe with a SDR-21 rating have the same pressure rating, such as the common 200 psi (Class 200). Other common sizing is SDR-32.5 Class 125, SDR-26 Class 160 and SDR-13.5 Class 315.
Class 315, Class 200, Class 125, etc.: The "pressure class" distinction tells you that the pipe is designed to withstand normal pressures 315 psi, 200 psi, 125 psi, etc. The industry standard is to have a built-in safety factor of 2:1 for pressure surges, so a Class 200 pipe could probably withstand 400 psi water hammer (shock) pressures.
Schedule 40/80: The "Schedule" distinction relates to actual wall thickness, not pressure class. As pipe size increases, the relative strength of Schedule pipe decreases; for example, 2" Sch.40 pipe is a higher pressure class than 8" Sch. 40. Sch.80 is gray in color and slightly thicker, and is used primarily for threaded fittings.
PIP, Sewer Pipe and Other Types: Other types/sizes of pipe (except Polyethylene, described below) are not designed for pressurized irrigation systems and should not be used. Try not to use some goofy type of pipe that you "got from a friend", etc.
