In the broadest of generalities, forastero beans have more of the bass-heavy, "chocolatey" notes and take a longer, hotter roast, but they tend to be somewhat "flat" or uncomplicated, with very little finish or development of the flavor on the palate. The vast plantations of west Africa that produce most of the world's cocoa beans (even though the species are native to central America) are planted in forastero monoculture and that's what most of the mass-market commercial chocolate you've ever had uses.
Criollo are lighter, fruity beans with all kinds of top notes and complexity, but typically too tangy for my tastes on their own, and lack the deep richness that we associate with chocolate. These are the beans that will burn to gross if you look at them funny.
Some trinitarios manage to achieve a desirable balance of these characteristics, but much of the art of chocolate-making is in blending different beans to balance each other's weaknesses.
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