First things first, let's define scale as it's used in reference to model railroads: Scale is the proportion to an actual size train or its prototype; the smaller the scale, the more you can fit into your allocated model train area.  

G Scale - 1:22.5 - G scale is most often used in garden layouts and is sometimes referred to as an LGB scale. It's ideal for those who appreciate gardening and landscaping.  

O Scale - 1:48 - Although similar to the O27 scale, there are a few differences which we'll outline below. If your goal is to keep the proportions as realistic as possible, you should consider a large area to build around this scale. The trains are bigger than all other scales except for G scale, and incorporate DC currents. 

O27 Scale - 1:48 - Similar to O scale, O27 is differentiated by the curves in the tracks. The O 27's area is a 27" arc or circle while the standard O Scale will encompass 31". Another significant difference is the 27's shorter and thinner rails. This is a particularly popular scale since there are so many accessories available that are life like and have working parts.  

S Scale - 1:64 - It's been said the S scale isn't quite as popular as many of the others; however, many of us are sentimental and are drawn to this particular scale since it's what we're most familiar with from our childhoods.  

HO Scale - 1:87 - Another popular scale, the HO is 1/87th in terms of comparison to the real thing. Its size is considered ideal and easily works on standard plywood sizes. This is another scale that offers a lot in accessories.  

TT Scale - 1:120 - Many tend to avoid this scale simply because suppliers of accessories aren't plentiful.