Here's how I understand it so far:
You can forecast the alcohol content by using the specific gravity reading, which is simply the measure of sugar in the wort. Using a hydrometer, you take readings once the wort is cooled. This is the Starting Gravity, or Original Gravity. There are also methods to forecast the potential sugar production of a given grain bill or extract quantity, Check out How to Brew at http://www.howtobrew.com/section1/chapter1.html
) When fermentation is done, you can take another reading, for the finished gravity. Using some math (which escapes me since I have an excel doc at home that does it for me) you can compute the Alcohol by weight and volume, in approximation. The way to controler alcohol content is by the amount of Malt or fermentable sugar you add to your brew. With All grain methods, technique and experience play a big role in sugar extraction efficiency, and therefore, potential alcohol production. In extract brewing, not so much, since the malt extract is basically the product of a grain mashing.