Light roasts are roasted for the least amount of time. Lightly roasted beans generally reach an internal temperature of 356°F – 401°F, right after the first crack occurs. These beans have less oils on them because they haven't been roasted at a high enough temperature.
The light roasts have the most caffeine (by volume) and the most acidity. They can have a different taste profile because the shortened roasting process prevents some chemical changes from occurring inside the bean. Origin flavors of the bean are more recognizable in light roasts since the flavors that come from the roasting process often aren't prominent. The acidity in light roasts is often accompanied by a citrus or lemon tone which some people find pleasing to the palate.
Medium roasted coffee reaches internal temperatures of 410°F-428°F. This is after the first crack and just before the second one occurs. They have a little bit more body than a light roast and less acidity.
Medium roasts are what the average American coffee drinker is used to. These roasts are considered to have balanced flavors. The acidity and body of a medium roast can vary but are usually somewhere in the middle. Some examples of medium roasts are House blend, Breakfast roast, and American Roast.
Beans roasted to medium-dark reach an internal temperature of 437°F – 446°F. This is during or just after the second crack. This roast will also start showing the oils on the beans' surface because the temperatures are high enough.
These roasts have a richer, fuller flavor, more body, and less acidity. Vienna Roast and Full-City Roast are some examples of a medium-dark roast coffee blend.
The roasting temperature for a dark roast is between 464°F – 482°F. There are visible oils on dark roast beans. Typically you cannot taste any origin flavors in a dark roast, just the effects the roasting process has on that type of coffee bean.
Dark roasts have sweeter flavors because the sugars in the coffee beans have time to caramelize. The longer roasting process helps it to develop a richer flavor and full body, which often leads to it having a buttery finish. They also have the least acidity of all coffee roasts. Dark roasts have the least amount of caffeine because they're roasted the longest. French roast is considered the darkest roast and has a pronounced smoky flavor. If coffee beans are roasted longer than a French roast (482°F), the oils and sugars in the bean will burn. Dark roasts often have European names because of the popularity of dark roasts in Europe, such as Italian roast.