Gearing is reffered to by ratios, either by saying the number of teeth on the chainring and cog, or by just saying the ratio of chainring teeth to cog teeth.
To find the gear ratio, just divide the number of teeth on the chainring by the number of teeth on the cog.
For example, 44-16 (44 divided by 16) is 2.75. If you ran 44-16, you could state those two numbers or you could just say that you run a 2.75:1 gear ratio.
Without taking into consideration the points that ettsn brought up, you could change the size of your chainring and, as long as the size of the cog changed accordingly, your gear ratio would stay the same. A 25-9 would be 2.78:1(rounded, plug it into a calculator and you'll see the number is long).