A simple explanation:
Gear Inches Explained @ Bike Gremlin.
Basically: (teeth on front chainring / teeth on rear cog) * diameter of wheel = gear inches. Or, roughly how far you travel for one revolution of the crank.
So, for example, on a 3x8spd 1990s MTB:
- High gear = (42 / 11) * 26 = 99 gear inches.
- Low gear = (22 / 28) * 26 = 20 gear inches.
- Still lower gear (ie, with a larger cassette) = (22 / 34) * 26 = 16 gear inches.
Here is a useful tool, where you can plug in the specific chainring and cassette components your bike has, and you'll get the numbers calculated for you:
Bicycle Gear Calculator @ Gear-Calculator.com.