"S" was the name given by the National Model Railroad Association (NMRA) in 1943 for modeling in a ratio of 1:64. Meaning, real-world measurements are divided by 64 to provide the scale model dimensions. The scale is also known as "3/16th scale", because a prototype English foot is equal to 3/16th of an inch in S-scale. Conveniently, each 1/64" tick mark on a ruler represents one scale inch on the model. This makes modeling in S-scale very convenient.
S-scale sits in the middle of all the popular model railroading scales, as shown in this table. Also shown are the conversion percentages to convert S-scale models to the other scales, or to convert the other scales to S. Use these with copiers or graphics programs, for example.
S to ...
... to S
Many people consider S-scale to be the perfect model railroad scale, because it doesn't take up much more space than HO-scale, but it does provide the heft needed in locomotives to make them run better. As modelers get older, the fine details in N- and HO-scale modelers may become harder to see, and working in those scales may become more difficult. S-scale makes working on your cars and engines much more fun. The larger O-scale and G-gauge would make working on those details even easier to see, but because those details are clearly visible, they almost have to be modeled. This makes modeling in those scales a lot of work. Further complications ensue when dealing with multiple or larger motors inside those engines, or with wanting to build full-size structures in our limited spaces. S-scale offers the best compromise.
The amount of materials and size of those materials needed to build models in the various scales goes up as the scale increases. However, this is offset a bit by market demand. This is why models in Z-scale can be more expensive than a similar model in HO-scale, purely due there being more people modeling in HO than Z. S-scale's costs are actually very comparable to equivalent HO-scale models. Prices goes up significantly when looking at O- and larger scales.
S-scale has also been called the "builder's scale". This used to be very real in the early days, but much less so today. Many items are available ready-to-run, or in kits that are easy enough to build. Nevertheless, if you enjoy building, "S" is a good scale to be in. Many people purposely switch to S-scale, precisely because there isn't as much available as there is in N- and HO-scale. It helps you keep control over your hobby budget, and it also challenges you to learn some new skills that the other scales don't require. S is something different, something new, something challenging, something fun, and brings that hobby spark back to life!
Ultimately, if you are interested in the hobby of model railroading, it is an individual decision as to which scale to use. We feel that S-scale just looks right to the eye, and, from those who have switched from N- or HO-scale, the feedback has consistently been that it is much easier to build things in S-scale, from track to structures and even scratchbuilding cars and engines.
The photo below is a great shot of comparing a similar 40-foot box car across the various scales.
In the photo below, note how the volume of the On3 car (back row, center) is almost the same as the S standard-gauge car (middle row, left), and how the HO standard-gauge car (front row, left) is almost the same as the Sn3 car (middle row, center).