What are schedule 40 and schedule 80 pipes as used on ships.difference in the pvc type and the steel type?

This reference does a better job explaining things:

Difference between Pipes and Tubes (http://www.engineeringtoolbox.com/pipes-tubes-d_347.html)

But basically schedule is related to wall thickness. The lower schedule, the thicker the wall. The outer OD is the same.

There are other pipe properties that determine whether the pipe can be used in the application.