What is the difference between schedule 40 and schedule 80 steel pipe? Is it just wall thickness and if so what is it? Does the schedule of steel pipe just refer to wall thickness and is it always thicker the higher the schedule? Thanks

Thickness of the wall. The higher the number, the thicker the pipe. But you can have multiple wall thicknesses within the schedule size. Other properties change too.

See:
Carbon, Alloy and Stainless Steel Pipes - ASME/ANSI B36.10/19

There are a lot of difference