Scale versus gauge...
G Scale is a scale for model railroads and because of its size and durability is most often used outdoors in the garden, although many modelers have indoor layouts. It is not uncommon for people to start out with a circle of track around the Christmas tree, the track eventually being moved to the garden for year round usage.
SCALE: Describes the size of a model compared to the real thing. For example, "G" scale (on average) is 1:29 (1 to 29; 1/29 of life size; you'd need 29 of an item strung end-to-end to be the same length as the real thing)
GAUGE: Refers to the distance between the rails.
The rails on life-sized track are 4' 8-1/2" (56.5") inches apart (measuring the inside edges). Narrow gauge track has the rails closer than that- 36", 24"; many variations of narrow gauge exist.
How are the trains powered?
Locomotives can be either track powered (electric) or battery powered. Some modelers will also use catenary. Most large scale engines use track power, which can be converted to battery power.
Due to the large size of the locomotives it is also possible for them to be powered with live steam. The latter are usually fired by butane or alcohol.
Remote Control: In a nutshell there is DCC (digital command & control) and RC (radio control). RC can either control the engine itself or the track. There are advantages and disavantages to both types
Can you run live steam and track power at the same time?
Some live steam locomotives can be run at the same time as track powered engines, providing the wheels are insulated. As a general rule it's not something that is recommended for a beginner.
What is Live Steam?
Where to buy...