When we discuss different types of chocolate, we're usually talking about cacao percentage. "Cacao," in terms of what's in a chocolate bar, means what the manufacturer derives from the cacao bean -- that is, cocoa solids and cocoa butter. Cacao percentage means how much of the bar is cacao product, as opposed to sugar. The 70% bar I mentioned is therefore about 30% sugar.
That's a pretty hefty percentage of cacao. Milk chocolate must, according to FDA regulations, contain a minimum of only 10% cacao, as well as at least 12% milk ingredients. The milk and high sugar content gives milk chocolate a gentler, milder taste and smoother texture than that of dark chocolate. Milk is the sweetest, and stickiest, of chocolates.
Dark chocolate is deeper in color and more intense in taste than milk. "Sweet" chocolate is a type of dark chocolate that must have a cacao count of at least 15%.
Semisweet, its kissing cousin, has a much higher minimum percentage of 35%. The chocolate chips many of us grew up with are semisweet, as are a lot of dark chocolate bars enjoyed straight out of hand. Its gently intense flavor and relaxed attitude toward melting make it the home baker's friend.
Bittersweet chocolate is a wonderful contradiction in terms. Like "pianoforte," which means "quiet-loud" because the instrument can be either and both, bittersweet chocolate incorporates all the moody intensity of "bitter" chocolate, tempered by a certain human bare minimum of sweetness. Like semisweet, bittersweet chocolate must contain at least 35% cacao; usually it contains a great deal more, and is more intense and less forgiving in both flavor and performance.
Unsweetened chocolate is exactly what it sounds like. It's never consumed as is. Which is a shame, since it would be a health fiend's dream -- all the antioxidants, none of the unhealthy refined sugar -- but even chocolate purists can't tolerate its merciless bitterness. The closest I've come is a few bites of an 88% bar, which is also the closest I've ever come to questioning the necessity of chocolate in my life.
In terms of health and flavor, remember that the higher the cacao count, the lower the percentage of sugar; and that milk may make chocolate smooth and sweet, but it also cancels out the antioxidant benefits and much of the intensity of the chocolate experience.