The best dark chocolate contains chocolate liquor, sugar, cocoa butter, vanilla, and lecithin (an emulsifier). It's sometimes called "plain chocolate" because it doesn't have milk as an additive.
There is no legal, manufacturing definition of what makes a chocolate "dark", but in general it contains a higher percentage of cocoa solids and less sugar than milk chocolate. It's usually distinguished by the percentage of cocoa solids in the bar.
The higher the cocoa content, the darker the chocolate.
For those looking to buy dark chocolate, in commercial chocolate bars the cocoa content can range from 30% (sweet dark) and up to 80% for extremely dark, high-quality bars. The cocoa content ratio favored by gourmet bakers is between 35% and 50%. European varieties are typically darker than American-manufactured chocolates.
Bittersweet and Semi-sweet chocolate also fall into this category. Generally, bittersweet chocolate has higher concentrations of chocolate liquor than semi-sweet does. Each contains varying degrees of sweeteners.
While milk and white chocolate are more common than dark, this sumptuous and exotic chocolate is rapidly becoming more widespread in candies, sauces, and gourmet treats.
This may be due to the recent revelations about its health benefits. It's been recognized as having a substantial amount of antioxidants that reduce the formation of free radicals.
This chocolate has a stronger, more intense flavor than milk chocolate because it doesn't contain milk solids to compete with the taste.
While milk chocolate is the predominant favorite among children, many adults prefer a less sugary variety. Scientists theorize that this is because as taste buds age they become less sensitive, and cocoa is a more prominent flavor for adults.
There are some chocolate aficionados who are of the opinion that the best dark chocolate has a somewhat bitter taste that mellows with age and tastes best several months after it is manufactured.
Whether that's true or not, dark chocolate will always hold a sophisticated place in the hearts (and mouths!) of chocoholics everywhere.