Attenuation simply put is the measured amount of how much a yeast converts malt sugars to CO2 and ethanol. All your yeasts will include an average attenuation because there are several factors that could change the attenuation of the yeast being used. The largest factor is the amount of sugars you stat with when you pitch it in your wort.
We all measure this (if we haven’t had too much to drink by the time it comes to measuring) by checking the OG of or wort and are FG once the yeast has all but finished it’s cycle weeks later.
A low attenuation is typically between 65%-70%, a medium is 71-75% and a high one being 76%-80%. Here’s the math for how to actually do that (From FG = OG – (OG x %) => % att. = (OG-FG)/OG) but if you’re like me, you leave the math to the pros and just get busy making delicious liquids.
Attenuation Chart http://byo.com/resources/yeast
Low attenuation yeast http://www.whitelabs.com/yeast/wlp002-english-ale-yeast
Medium attenuation yeast http://www.danstaryeast.com/products/bry-97-american-west-coast-beer-yeast
High attenuation yeast http://www.northernbrewer.com/shop/saflager-w-34-70.html
Photo: Daniel Spiess