I think it is all model railroading, but there are sub-groups from there with sometimes fuzzy descriptions and sometimes over-lapping interests.
At one time Hi-rail would have described someone with a goal of realism. now it would be more 2-rail scale/3 rail scale that I think folks into exacting detail and realism use to describe them selves. As for folks that just like to run trains on track and don't give a hoot for the scenery or if their trains look realistic, I think most folks around here group that into 'a toy train layout' or 'post/pre war style'
The names may also change depending on scale/gauge that you're referring to. 'O' has a long history of being a more toy like product for folks that like to run the trains and do things with them, where as N or HO have typically been more about getting things to look realistic. With better electronics in smaller scales, and better detail on larger scale, that has all started to bleed together now days.
A good place to start might be to look at the descriptions for each of the main forums here on OGR.
JGL
Edit: Oh and for the last bit, when someone builds a fairly exact model of an actual, real site, say a switch yard or some such, it is called a diorama