Many of the chocolate bars found in stores today have a cacao percentage on the label. This refers to the percentage of the bar (by weight) that is derived from the cacao bean.
The two main ingredients in a chocolate bar derived from the cacao bean are "chocolate liquor" (a.k.a. cocoa mass) and "cocoa butter." For example, Equal Exchange's Organic Very Dark Chocolate bar has a 71% cacao content. This means that 71% of the bar is chocolate liquor plus cocoa butter and 29% is other ingredients (e.g. sugar and vanilla).