For real world trains, narrow gauge refers to track which has less than 1435mm (known as Standard Gauge) between the rails. Examples include, but are not limited to, Puffing Billy, the Ffestiniog Railway (Wales), the Talyllyn Railway (Wales), and the 1067mm (3’ 6”) railways in QLD, WA, SA, and TAS.
For model trains, this depends on your scale. Often the track used for narrow gauge in one scale is the track for the scale size below it. For example, HO scale narrow gauge trains usually run on N gauge track.
Narrow gauge models use a combination scale code. The three most popular are On30, Hon30, and OO9.
On30: O scale, narrow gauge, 30” (2’ 6”) gauge for the prototype
Hon30: HO scale, narrow gauge, 30” (2’ 6”) gauge for the prototype
As with standard gauge models, the British do things differently.
OO9: OO scale, 9mm (N scale) track gauge
Narrow gauge has the advantage of being able to realistically fit a lot of track and scenery into a small space. They were built with tight corners and to travel over rugged terrain, so a small layout will look like the real thing!