What exactly does roasting do? The sugars, fats and starches that are within the bean are emulsified, caramelized and released. This creates the delicate coffee oil. This oil is what gives coffee its distinctive aroma and taste.
Beans that have been over-roasted will take on a burned, smoky or charcoal flavor. Also, there is less caffeine in the darker roasted coffees than in the lighter ones. The roast alone doesn't determine the resulting coffee taste or quality. The origin of the beans and detail to harvesting, processing and drying make a big difference. A bean from Ethiopia will taste differently than a bean from India, even if they are both roasted exactly the same.
Medium light brown beans. This roast is the norm for eastern USA. This roast (and sometimes cinnamon as well) is the most often used for cupping or professional tasting.
Medium dark brown beans. The beans will start to show some oily drops on the surface with this roast. Full City will have caramel or chocolate undertones.
Beans are starting to get dark brown, and French roasted beans are shiny with oil. There is less acidity, but with burned undertones. This roast is often used when making Espresso. Many people think this is the darkest roast available, but that's not true.
Similar to regular French, but more so. Darker and oilier looking, and with a stronger burned flavour.