NPT is talking about the threads, National Pipe Tapered is how I remember it.
Pipe is described by a nominal size and schedule. The wall thickness is the "schedule" and probably has to do with a pressure rating because it increases as pipe diameter increases.
For example, 1" pipe is 1.315" OD. Schedule 40 wall thickness is 0.133". Schedule 80 is 0.179".
4" pipe is 4.500" diameter. Schedule 40 wall thickness is 0.237" Schedule 80 is 0.337".

Over 12" diameter, pipe is OD x wall thickness.

Metallic tubing is OD x wall thickness.

It's handy to have a couple of books around when you're doing this stuff.
R.