Lightly roast coffee beans go through overall lower temperatures in the roaster. They have soaked up less energy which allows them to keep more of their acids as well as their fragrance. As a result, they have increased levels of acidity and fragrance to them. Lighter roasts typically have much less physical body and are typically less sweet, although a few are very sweet and full-bodied.
These coffee beans rest somewhere between the Darker and Lighter roasts. There are numerous shades of Coffee roasts in this category, but they are typically milder versions of either darker and lighter roasts. Many of the roasters in use today focus on the medium roast as it is good for both espressos and filter coffees.
Darker roasted coffee beans go through greater levels of heat in the roaster. Consequently, they tend to have very little levels of acidity and as a consequence might taste unplesant. But sometimes, in experienced hands, they can be made to be very tasty. Typically, darkly roasted coffee beans produce coffee that has more body and a sweeter taste.