From John Palmer's How to brew:
Attenuation: This term is usually given as a percentage to describe the percent of malt sugar that is converted by the yeast strain to ethanol and CO2. Most yeast strains attenuate in the range of 65 - 80%. More specifically, this range is the "Apparent" attenuation. The apparent attenuation is determined by comparing the Original and Final gravities of the beer. A 1.040 OG that ferments to a 1.010 FG would have an apparent attenuation of 75%.
(From FG = OG - (OG x %) => % att. = (OG-FG)/OG)
The "Real" attenuation is less. Pure ethanol has a gravity of about 0.800. If you had a 1.040 OG beer and got 100% real attenuation, the resulting specific gravity would be about 0.991 (corresponding to about 5% alcohol by weight). The apparent attenuation of this beer would be 122%. The apparent attenuation of a yeast strain will vary depending on the types of sugars in the wort that the yeast is fermenting. Thus the number quoted for a particular yeast is an average. For purposes of discussion, apparent attenuation is ranked as low, medium, and high by the following percentages:
65-70% = Low
71-75% = Medium
76-80% = High
My last beer, Brewpastor's 777 rye, got over 90% attenuation. Shocked me, it did! But I mashed at a low temperature, and used nottingham yeast. But your yeast does have an attenuation range. See this website: http://www.wyeastlab.com/beprlist.htm
to get an idea of what you can expect from your yeast.
What this means in practice to us homebrewers is "don't bottle until your yeast is done" and how we can make maltier or dryer beers, depending on style. If you want a maltier, sweeter beer, you can usually pick a less attenuating yeast. If you want a drier beer with less body, use a more attenuating yeast.