chocolate substitutions in baking brownies from scratch
Chocolate - one of my favorite subjects.

The rules for naming chocolate (milk, semi-sweet, bittersweet, dark, extra dark, etc.) vary a bit depending on where you live.

However, as a general rule you may substitute semi-sweet chocolate for dark chocolate in a brownie recipe. Semi-sweet chocolate doesn't necessarily have a strict cocoa level requirement, though some companies will list the cocoa rating (in terms of percent cocoa) on it. If your recipe calls for "extra dark" chocolate, bittersweet chocolate may usually be used as a substitute.

In the US, there is no legal distinction between semi-sweet and bittersweet chocolate, so you should check the package and see if it lists a cocoa percentage. The higher the cocoa percentage, the less sweet the chocolate and the "darker" (i.e. more chocolate and bitter flavor) the chocolate. For most recipes that call for dark chocolate, I'd recommend looking for a chocolate in the 50-60% cocoa range. Ghirardelli's dark chocolate is 60% cocoa in the US, I believe.

Unsweetened chocolate cannot be used as a substitute for another other type of chocolate without the addition of sugar, and the ratios can be difficult to get right, so I do not recommend it.

The wikipedia page for "Types of Chocolate" actually has one of the best summaries that I've seen, including charts based on your region (US, Canada or EU - apparently no-one eats chocolate elsewhere). Link is below. I would provide other, more official sources, but very few of them summarize it as well. I will add some links if I can find good ones.

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