Model railroading is a hobby shared by many people throughout the world. If you haven't figured it out already, model railroaders try to mimic real life railroads by creating miniature models in their free time. Track is laid on a 'layout' and the trains are run in circles or patterns. Fortunately, it gets much more complicated than that; scenery is created, tracks are run all over the place, and electricity is wired in.
Scale vs. gauge
These miniature trains come in many different sizes; and there is a difference between scale and gauge:
HO scale is the most popular, and is built at 1:87.1 of the prototype. When written alone, it signifies standard gauge. HOn3 is a narrow gauge, still at the 1:87.1 proportion, but the distance between the rails is smaller. HOe, HOn2, HOn2 1/2 are all narrow gauge railroads, but again, the scale remains at 1:87.1.
Generally, the smaller the scale, the more you can fit in any given area. But take into consideration the size of the parts you'll be working with, as well as the cost. It is because of the all-around characteristics that I recommend HO scale to anyone who is absolutely new to the hobby. There is the widest product line available, and a large amount of action can fit into a tight area without making the parts too small to handle. But if you're convinced HO isn't for you, then I suggest talking to a hobby store clerk.