In general, there are three main levels of coffee roasts - light, medium, and dark. There can also be some middle ground, like a medium-light roast, or a medium-dark.
In general, there are a few rules of thumb:
Light roasts are a light brown color, with a light body and none of the natural oil on the surface of the beans. They have a toasted grain taste and pronounced acidity compared to darker roasts. The original flavors of the bean are retained to a greater extent than in darker roasted beans.
Light roasts also retain more of the caffeine.Light roasted beans reach an internal temperature of 180°C - 205°C (356°-401°F). At around 205°C, the beans will pop or crack open and expand in size. This is known as the "first crack", and light roasted coffee generally has not been roasted beyond this point.
Medium roasts are medium brown in color, with more body than light roasts. Like the light roasts, they do not have natural oil on the bean's surface. Medium beans lack the grain flavor present in lighter roasts, and have a more balanced flavor, aroma, and acidity. Caffeine is somewhat less than light roasts, but there is more caffeine than in darker roasts.
The beans are roasted to the end of the first crack, stopping short of the second crack, roasting to about 215°C (419°F).
If you're not sure which roast you like, you can't go wrong with a medium roast.
Medium-dark roasts have a richer, darker color with some oil starting to show on the surface of the beans. Medium dark roasts have a heavier body than medium roasts, and slightly less caffeine.
The beans are roasted to beginning or middle of the second crack - about 225°C (437°F) - and the flavors of the roasting process will begin to come through at this level.
Dark roasts are dark brown, sometimes black in color. They have a thin layer of oil on the surface, which is often evident in the cup when dark roast coffee is brewed. The coffee's original flavors and notes are surpassed by the flavors of the roasting process. The coffee will generally have a bitter and smoky, or even burnt taste, and the amount of caffeine is substantially diminished.
For dark roasts, coffee beans are roasted to an internal temperature of 240°C (464°F), about the end of the second crack or beyond. They are rarely roasted past an internal temperature of 250°C (482°F), as at that point, the beans taste is dominated by that of tar and charcoal.