DOM = Drawn Over Mandrel, a cold roll-like process. Very tight tolerance tubing. Usually high in carbon as well, if my memory serves right. I have a variety of the stuff, from .5 inch to 2.5 inch. I use it for making threaded bungs, chassis stuff, roll cages, etc. You can get, for instance, 1" od DOM tubing with a 1/4" wall. Well, it comes perfectly ready to tap out to 9/16 coarse thread. I make shock bungs with this size. I also have 1" od DOM that has a .188 wall that comes ready to thread to 3/4 fine thread. I use this to make 4 link and ladder bars, because of the heims that thread into the ends.
If you had 1" od dom with a 1/4 wall, what they do is draw non-molten steel over a 1/2" mandrel. It is cold drawn, so there is no warping or stress relief.
It is also known as seamless tubing, because it is not electrostatically welded, as pipe and normal tubing are. Only problem.....it is expensive. you can buy 1" .125 normal tubing for about a buck a foot, whereas the same thing in DOM will run about 3 bucks a foot.
NPT is the acronym for National Pipe Thread. Used primarily here in the states, I believe. Whoever came up with this measuring system was either blind or never calibrated their tape measure. Pipe comes in schedule measures. Instead of saying "wall thickness", you say "schedule" Generally, the lower the schedule, the thinner the wall. Schedule 10 is right around sheetmetal thickness, Schedule 40 is usually .1" to .188 wall, and Schedule 80 is generally .188 to .375, depending on pipe size.
It is a screwball system. You buy 1" schedule 40, it has just about 1/8"wall. If you buy 4" schedule 40, it has 3/16" wall.
The crazy thing is, pipe size is supposed to go by the inside diameter, but the ID is never that exact number.
3/4" schedule 40 is like .8 ID, and around 1.05 OD.
1" schedule 40 has a ID of 1.1 give or take, and a OD of 1.315
So, 1" NPT is more like 1 3/8"
1/4" npt is almost a 1/2" hole
and so on and so forth.....