BV = Biological Value.
It's really the only measure of protein quality that is meaningful for athletic types. It's an index that represents how efficiently protein is retained by the human body. When the BV system was first introduced, since all other foods scored lower than the whole egg, the whole egg was elected as the "standard to beat" ? 100%. All other foods, whose protein content was not as efficiently absorbed, earned lower percentage values. Here are some BV values that are earned by different foods (in descending order):
110-159 - Whey Protein isolates blends
104 - Whey Concentrate (lactalbumin)
100 - whole egg
91 - cow milk
88 - egg white
83 - fish
80 - beef
79 - chicken
77 - casein
74 - soy
59 - rice
54 - wheat
49 - beans
As an example of how this affects your food choices, imagine that you need 25g protein for you post workout recovery meal.
-Since each raw large egg supplies 6.25g of protein (ref: USDA). Four whole eggs would then fill the bill. Now, since whole egg has a BV of 100%, you body will get the absolute most out of its 25g of protein.
-Since raw chicken breast supplies 6.60g of protein per ounce (ref: USDA), you would need 3.79 oz of raw chicken breast to equal 25g of protein. But since chicken has a BV of 79%, the body needs more meat in order to provide an equal amount of absorbed protein (specifically, 4.80 oz).
The lower the BV for a particular food, the more you'll need to eat to hit your protein target, BUT just keep in mind your total caloric consumption as well.
I should note that since this efficiency scale was adopted, protein sources have been created that offer higher levels of protein absorbability than whole egg. As a matter of fact some of these newer sources offer as much as 150% the protein quality that whole egg boasts (giving it a BV value of 150). Whey concentrate has a BV of about 104 (Optimum Nutrition's 100% Whey only has a BV=100). Whey peptide blends start at about BV=110, and run as high as 150 or so.