When I started to build my shop and all the work supports, tables and other equipment I was mostly getting the steel from the scrap yard. I just picked up pipe and tube which was “about the right size.” It wasn’t until much later and working on commission jobs that I seriously studied pipe and tube characteristics.
As it turned out, I needed both in different circumstances. Pipe is a vessel which is designed to conduct a fluid. It is sized by inside diameter and the wall thickness varies in accordance with the pressure required to contain the fluid. All of that was essentially irrelevant to my needs.
Usually, what I needed was tube. Tube is considered a structural steel element and is sized by outside dimension. The wall thickness varies according to the structural load it will bear. In the sizes which I work with, tube is less expensive than the roughly equivalent size pipe.
I don’t have an engineering background so I came to blacksmithing lacking a lot knowledge which would have been very useful and which I acquired piecemeal and haphazardly along the way.
There is a lot more to consider when building with these materials including issues of threading, fitting, ASTM grade, seam or seamless and weight. Here are some references which may be useful.
Pipe vs Tube
Carbon Steel Pipe
Structural Steel Specifications in PDF
Pipe Weight Chart
Tubular Steel PDFs