There are four common scales: 25mm, 20mm, 15mm and 1/300th used for land warfare.
25mm is the largest and allows beautifully detailed figures but the figures are expensive and are only really suited to skirmish games because of the amount of room required. Long a staple of fantasy and historical gaming WW II miniatures are fairly new in this scale.
20mm is the scale for WW II wargaming because it allows cheap 1/72 scale vehicle kits to be used. It is just about the same as the OO/HO scale model railway scale.
15mm is smaller and cheaper than 20mm. It is commonly used for historical wargaming - such as Napoleonics or the American Civil War.
1/300th or 6mm (1/285 in America) is the second WW II scale. The vehicles and figures are tiny -even a Jagdtiger is less than half an inch long - but they are cheap and a lot can be fitted onto a tabletop.
There are also many other scales:
10mm is fairly new. The Games Workshop Warmaster figures use this scale - I suspect largely to prevent players from using the cheaper 6mm ranges available. This is the same as 'N' Gauge model railways.
1/1200th This is the common standard for pre-20th century naval wargaming or WW II coastal warfare. It is the smallest scale at which recognisable aircraft models are available.
1/3000th This is the common standard for 20th century naval wargaming.
1/6000th This is a new naval wargaming scale pioneered by Hallmark models. It is cheap and allows a more realistic relationship between ground and model scale.