Gauge refers to track size. Scale, on the other hand, measures the size relationship between a model and its real-world prototype. For example, a Lionel locomotive that is 1/48th the size of the real thing is called 1/48th or 1:48 scale. (As it happens, O gauge trains are 1/48th scale.) Sometimes the terms "gauge" and "scale" are used interchangeably even though, technically, they're different.
Track layouts vary but will usually be one of 2 types or a combination of the two: Continuous Running (or loop to loop), and Point to Point. After developing your concept, or theme, you're now ready to consider various model railroad track plans.
When laying curved track, you should use an easement, which eases your way into and out of the curve. In other words, start your curve with a larger radius, then ease into the tighter radius, then come out of the curve again with a larger radius. Your trains will look better going around the curve that way.
You can use turntables, wyes or reversing tracks as a way of turning your trains around (without having to manually lift them up off the tracks). A passing siding is an important component of any model railroad track plan. A conventional siding is a simple way of getting one train around another. A Spur is any short branch off the mainline that can be use to service an industry, station or other facility.
The best part of model railroading is the plotting and planning of your ultimate layout and equipment, and enjoying the many hours of dreaming about it.