Right, all right-- ah, let's begin at the beginning. All of those are magical arts, yes, but potions are about brewing liquid magic. Herbology has to do with magical plants, yes, and how to extract magical qualities out of regular ones. As for that-- well, even if I were to explain to you how precisely to do a potion, you wouldn't be able to, because you need magic to-- oh, what's the word-- to give it an extra boost, I suppose. To evoke the magical properties within it. Does that make sense?

As for Dark Magic-- well, yes, it's as dark as the name suggests. It's anything cruel or malicious-- how to kill someone, or control their mind, or cause them pure pain. How to transform them into an animal. Et cetera.