Attenuation is a measure of how much of the sugar in the wort has been fermented by the yeast.
In home brewing, we usually deal with apparent attenuation:
apparent attenuation = ( ( OG in points - FG in points ) / OG in points ) x 100
Apparent attenuation is different (higher) than the actual attenuation, because the alcohol produced during fermentation is lighter than water, and throws the reading off.
The apparent attenuation you will get on a given batch depends on a number of factors, including the types of grains/extracts used, mash temperature (if brewing all-grain), and the strain of yeast. Typically, apparent attenuation will range from 65 to 80%. Beers with a lower apparent attenuation will be sweeter, fuller-bodied, and lower in alcohol. Higher apparent attenuation will result in a drier, thinner, higher alcohol brew.
What is the apparent attenuation of a beer with an OG of 1.055 and a FG of 1.016?