Coffee Roast Levels
Roasting takes the bean from its green state (pale yellow or green - smells almost like grass) to its final brewable state. Roasting processes will be covered in a different blog, however the roasting time and temperature profile control the bean's final roast level. The more time at temperature the darker the roast.
The Art & Science of Coffee Roasting
Light Roast
The earliest roast state is after the "first crack" (literally sounds like popcorn popping - but more on that in the Roasting Processes post. at this roast level the beans are cinnamon brown. Light roasts also have the highest caffeine content and display more of the origin flavors that come from the growing area. Minerals in the soil, climatic conditions, water, as well as other plants growing in the area contribute to the "roast notes" that are detectable as various fruits and/or chocolate. Light roasts tend to be more "bright"(like a circus fruit is tangy but milder).
Medium Roast
Medium roast is in between the first and middle of the second crack (yes there is a second crack in addition to the first - this one sound like an unnamed rice cereal in a bowl of milk that you may remember from Saturday morning cartoons - again, more on that in the Roasting Blog). The beans are darker and once the beans have cooled off may have droplets of oil seeping out. The brew now has lost some of the "brightness", but is smoother and you'll notice it has lost fruity notes generally. Flavors of chocolate can be more pronounced.
Dark Roast
Dark roast is when the beans are taken all the way through the second crack. Not all beans are meant to be roasted dark. Most tend to loose all of their origin characters at this point and can taste similar to one another. Sumatran coffees however are better roasted dark. The beans now are... well dark in color and may have an oily sheen. A little longer and you get an espresso roast.