If you have a look at the calculators link you'll find an ABV calculator. Alcohol content is calculated from the drop from the OG (Original Gravity) to the FG (Final Gravity)...this is how much sugar has been converted to alcohol.
OG is the amount of sugars in your wort - whether they're from sugar, malt extract or extracted from grains by mashing
FG is what's left when the yeasties are finished - it can vary according to the yeast strain, some have higher attenuation than others (attenuation is how much of the available sugars they'll convert - expressed as a %), temperature, pitching rate (how much yeast), the types of sugars in the wort and how fermentable they are