Also called bitter, unsweetened, or cooking chocolate, this is the stuff everyone seems to have sampled once accidentally in his youth. It looks and smells delicious, but as it contains no sugar, tastes pretty awful until incorporated into a baked good.
Baking chocolate is made of pure chocolate liquor (100% cacao) combined with enough of some sort of fat to form into solid bars. Sugar, vanilla, and lecithin are added to this in various proportions, to create chocolate bars which may also be meant for baking, but are NOT what's meant by the term "baking chocolate." When a recipe calls for this, it means this, and you can't substitute any other sort of solid chocolate without putting the ratios all out of whack and potentially ruining your dessert. (This would be true even if US labeling regulations for chocolate were not so inexact - the cacao content of things called "bittersweet" can differ widely between brands.)
However, in a pinch, you can use 3 tablespoons cocoa powder plus 1 tablespoon of butter, margarine, or unflavored vegetable oil to replace an ounce of baking chocolate. This may affect texture, so don't do this unless you have to.