All real chocolate comes from the cocoa bean, the fruit of a tropical tree, Theobroma cacao. Much of the quality of the chocolate will depend on the origin and quality of the beans.
To make chocolate, processors roast and shell the cocoa beans, leaving only the centers, called nibs. These nibs are then pulverized or ground into a smooth liquid that's called chocolate liquor (although it contains no alcohol). When the chocolate liquor cools, it forms solid blocks.
Chocolate liquor is the basis for all things chocolate. Pure chocolate liquor is very dark and bitter and has only two components—cocoa solids and cocoa butter. The solids give chocolate its characteristic dark, strong flavor, and the cocoa butter translates to a smooth mouth feel.
In its natural state, chocolate liquor contains a little more than half (50% to 58%) cocoa butter and the rest solids. Early on, producers learned that by increasing the cocoa butter, they could create chocolate with a better sheen and smoother texture. So they developed a high-pressure filter process that breaks down chocolate liquor and separates the solids from the butter. They could then manipulate the chocolate to produce a range of styles.
To create eating chocolate, sugar and flavorings are added to the cocoa butter and solids. While some sugar is needed to make pure chocolate palatable, the best examples contain a high percentage of real chocolate and only small amounts of sugar or other additives.
This last detail is perhaps the most confusing when it comes to deciphering chocolate labels. When manufacturers list the percentage of chocolate on a label (a practice common in Europe and gaining popularity here), they often use the terms "X% of cocoa solids" or "X% of cocoa." What they're actually referring to is the total percentage by weight of cocoa solids and cocoa butter combined, in other words, the total percentage of ingredients derived purely from the cocoa bean. The remaining weight of the chocolate will consist of sugar, lecithin (a soy-derived emulsifier), and typically vanilla. Lesser quality chocolates also include other fats (like palm kernel oil) and flavorings.
What these percentages don't tell you, however, is the proportion of cocoa butter to cocoa solids. About the only way to figure out whether one chocolate has more cocoa butter than another is to compare the nutritional labels. As long as you're comparing first-quality dark chocolates without any additives, the one with a higher fat content will be the one with more cocoa butter. This will most likely be the more expensive of the two as well, since cocoa butter is more valuable than the solids for its texture and richness. Also, check the ingredient list while you're at it, because if the chocolate contains any dairy products or other types of fat, this will skew the fat percentage.