HI Scott..
"Gauge" means the distance between the rails...nothing more.
"Standard gauge" is 4 feet eight and a half inches..thats what all the modern US railroads are running on.
anything narrower than that is "narrow gauge"..
3-foot gauge, 30 inch gauge, 2 foot gauge, and many other narrow gauges..
In the Large Scale model train world, there are models of both standard gauge and narrow gauge prototypes..
but! here is the tricky bit..they ALL run on the same "G-gauge" track! 45mm gauge.
If you look through an issue of Garden Railways magazine, you see the big modern diesels by USA trains, Aristocraft, and MTH..
these are all models of standard gauge prototypes, in either 1/29 or 1/32 scale..
There are also some models of standard gauge steam locos..
Such as the Aristocraft pacific and 2-8-8-2, MTH Hudson and Big Boy..etc..
LGB and Bachmann make models of mostly Narrow gauge prototypes!
these model locomotives and cars run on the exact same track as the big modern diesels,
but they are models of 3-foot gauge prototypes rather than Standard gauge prototypes..
generally 3-foot gauge prototypes..
Bachmann Spectrum locomotives are all 1/20.3 scale, models of 3-foot gauge prototypes.
LGB is more fluid..their models arent always exact to scale..but they can generally be considered 1/22.5 scale.
There is also some 1/24 scale out there too..
These three scales (1/20.3, 1/22.5 and 1/24) of narrow gauge trains are "close enough" that they can mixed and matched without any glaring visable problems.
The exact scales arent terribly important..they all run on the same "G-gauge" track..so track isnt an issue.
Most people pick either Standard gauge in 1/29 or 1/32 scale...the diesels and modern (1920's-1940's) steam.
Or Narrow Gauge..the 3-foot gauge earlier steam engines, 1870's-1900 era..
Of course, you cAN run both if you like!
I have a Bachmann spectrum mogul, 1870's 3-foot gauge.
and a set of USA Trains Alco PA passenger diesels, 1950's standard gauge,
both sitting on the shelf behind me as I type this..
and I plan to run both on my railroad! (when I get it built!)
they all run on the same track..you get into problems with scale when it comes to figures and buildings..
A building in 1/32 scale (if placed on a 1/32 scale model with modern diesels)
is significantly smaller than a 1/20.3 scale model of the SAME building! (if placed on a 120.3 scale model with 1870's narrow gauge trains)
start out by thinking in terms of standard gauge or narrow gauge..
which are you more interested in?
those are the main two "classes" of locomotives and rolling stock available..
Once you have that straight, you can get into more detail about all the different scales and where they came from and what they mean..
Scot