Dark Roast is the term for coffee, which has been darker roasted, thus giving a more bitter and less sour taste than coffee beans roasted for a shorter time.
When the coffee beans are roasted, the oil accumulates in small pockets through the heart of the bean. The longer the beans are roasted, the more moisture will leave the beans, and the coffee oils will evolve even more and appear to the surface of the coffee beans to a greater extent, giving these dark-roasted coffees their distinctive oiled appearance. The longer roasting time causes the bean to burn. This gives the bittersweet flavor, which lovers of the dark roasted coffee so much appreciate.
Dark roasted coffee contains significantly less oxygen and caffeine than the lighter roasted coffee, as these are burned during the roasting process.