OO / HO scale
OO is the most common scale in the United Kingdom. The scale is 4mm to one foot (1:76,2).
In Europe HO is the most common scale in use whith a scale of 1:87 (3,5 mm to one foot).
Both scales use the same gauge rails 16,5 mm.
The difference in the scales are not very big and most rolling stock can run on either type of layout. The most noticeable difference is that OO rolling stock is a bit bigger.
EM (eighteen millimetre) and S4 (scale four) use the same scale as OO but with more accurate gauges as 16,5 mm is to narrow for 1:76,2. EM use 18 mm gauge and S4 18,83 mm gauge.