Chocolate Percentages: what do those numbers mean?

Percentages on chocolates can be confusing.  The percentage refers to the amount of cacao in the chocolate.  In general here are some chocolate guidelines:

Semi-sweet chocolate is generally in the range of 52% - 62% cacao.

Dark Chocolate is generally at least 62% cacao. 

Bittersweet chocolate is usually in the range of 63% - 72% cacao.  It can go up to as high as 99%, however this is not a range of chocolate that people are generally comfortable eating. 

Milk chocolate, which as the name implies is chocolate to which milk has been blended, will be lablled in the 40% range.  It can go as low as 36% cacao.

White chocolate, which also contains milk, is eichincally not chocolate.  This is because white coacoalte does not contain cacao solids.  However, it is made with cocoa butter.