Milk vs. Dark vs. Semi-sweet vs. Bitter-sweet chocolate?
Milk chocolate is unique in that it contains a significant quantity milk, either in the powdered, liquid, or condensed form.

Dark chocolate is a category of chocolate that includes semisweet and bittersweet chocolate. The US FDA actually does classify dark chocolate as anything containing 35% or more cacao (liquor or butter). In practice, semisweet chocolate is typically much sweeter, and contains about 50% sugar. Bittersweet chocolate, on the other hand, typically contains about 33% sugar. Some dark chocolates may contain small amounts of milk, but not enough to be called milk chocolate.

Source: http://en.wikipedia.org/wiki/Types_of_chocolate