There are thousands of articles about railroad model scales so I will not write many words.

Just for quick reference, I plan to make models in the following scales:

Name Scale Description
G scale 1:22.5 Commonly used for big garden raylways.
Read more on Wikipedia: G scale
O scale 1:48 Not very popular but some manufacturers make models in this scale.
Read more on Wikipedia: O scale
OO scale 1:76.2 Probably the most popular scale in United Kingdom.
Read more on Wikipedia: OO gauge
HO scale 1:87 The most popular scale in the world (except U.K.). Almost all manufacturers produce large amount of models in this scale. If you did not decide in which scale to build a lyout then this is probably the best choice.
Read more on Wikipedia: HO scale
N scale 1:160 This is probably the second most popular scale worldwide. If you do not have a lot of space for HO scale layout then N scale is good choice. There are many rolling stock stuff, building and decorations in this scale.
Read more on Wikipedia: N scale