Your question is more reasonable than the answers you are getting.
In the USA, "metallic conduit", which I won't define other than as a raceway to carry "something", there is stiff stuff and flexible stuff. The stiff, which must be bent with some sort of tool (the Incredible Hulk notwithstanding<g>), exists as "EMT", "IMC", and "RMC" generally thought of as electric metallic tubing, intermediate metal conduit, and rigid metal conduit.
LONG AGO, just EMT and RMC were common to the plant where I hung around as a kid. We called them "thinwall" and "rigid".
Today, EMT is heavily used in commercial buildings to contain insulated (usually THWN) conductors from panelboards to outlets. In MOST residential, "NM" cable would be used for this application. EMT is USUALLY galvanized steel, but there are less common and more pricey options ... plastic coated, painted, and I THINK stainless. In the standard galvanized, it is inexpensive. In my area, awning manufacturers even use it for framework, welding (yeah, galvanized) and bending as required. A few cities have been discussed here as requiring it for residential wiring ... no NM or UF type cabling.
RMC is made to the same dimensions as Schedule 40 pipe with tapered male threads, (NPT, not BSPT to my knowledge) and couplings are straight ... thus difficult to make pressure tight ... note that electrohydraulics is my area of expertise, so I am familiar with the concept of pressure tight and that tapered pipe eventually probably won't be ... we don't use it over 1/4" pipe size unless required for some (dumb?) reason.
IMC is, AFAIK, a newer product. It has OD and threading the same as RMC (EMT is not threaded). It is thinner wall and lighter weight than RMC. It is made from higher strength steel than RMC, and has similar physical properties. For a heavily loaded mast, many feel it to be the product of choice. Certainly if one "size" larger is used than RMC, it is still lighter and the larger section makes it stiffer.
If I'm burying, it is legal, but "many" (no source cited) feel it is less tough than RMC as related to corrosion. The zinc thickness specification is the same. A quick Google search turns up
http://o.b5z.net/i/u/10069179/f/Conduit_Standards_and_Specifications.pdf from one manufacturer. The Steel Conduit Association,
http://www.steelconduit.org/qa_home.htm has more detail as well.
I "ain't no lectrishun", and this may be a little off, but not much.
George