What is attentuation? How does it effect beer flavor and quality? How is is calculated and why do I need to know it?
Attenuation is a measurement of a beer's drop in specific gravity during fermentation. It is expressed as a percentage of original gravity and calculated by (OG - FG) / OG. We measure apparent attenuation because the alcohol produced during fermentation is lighter than water meaning OG readings are not truly measurements of the sugars in beer.
Most often homebrewers use the apparent attenuation listed by their yeast supplier to predict final gravity, the stopping point of fermentation. Suppose you brewed a pre-prohibition lager with an OG of 11.4 ºPlato and chose WYeast 2206 (Bavarian Lager). WYeast lists the attenuation range at 73-77%. From this you know that fermentation will cease when the beer's FG is between 8.3 & 8.7 ºP.
Attenuation is a more true measurement of fermentation activity than anything else.
When designing beers you can use attenuation to influence the flavor of the finished product. The higher a strain's attenuation, the drier the resultant beer. Say, for example, you want to turn your favorite IPA, made with White Labs WLP001, into a Double IPA. Knowing that you need a maltier beer to balance the big hop flavor you could choose WLP008 which has a lower attenuation range, but similar flavor profile.
Attenuation is the percentage of fermentables actually consumed by your yeast. It's easiest to calculate "apparent attenuation," which is simply (OG - FG) / OG. This is called apparent because it does not take into account the specific gravity of the alcohol that has been produced.
Commercially available homebrewer's yeast always lists it's expected attenuation range. From this range, you can can calculate what your expected final gravity will be based on your starting gravity. It's important to know this because it's a way of making sure you don't bottle too early. If you bottled when airlock activity stopped without knowing the attenuation of your brew, you might find out that the bottling activity woke up some sleeping yeast, who then found plenty of yummy fermentables to eat in addition to your priming sugar, and you'll wake up one night to the sounds of exploding glass.