Could someone give me a quick chocolate primer? I'm confused about the difference between bittersweet and semisweet chocolate. I suspect that bittersweet = Baker's chocolate, which usually comes in bars, but I'm not sure. If anyone knows the cacao percentages, it would be great if you could tell me here.

Also, I've seen recipes in which it's said that it's okay to substitute semi-sweet chips for bittersweet chocolate. Are they totally interchangable or no? I imagine one would be a little bit sweeter than the other, but is the substitution appropriate for all recipes, or just for the ones specified?