At lighter roasts, the bean will exhibit more of its origin flavour; the flavours created in the bean by its variety, the soil, altitude, and weather conditions in the location where it was grown.
As the beans darken to a deep brown, the origin flavours of the bean are eclipsed by the flavours created by the roasting process itself. At darker roasts, the "roast flavour" is so dominant that it can be difficult to distinguish the origin of the beans used in the roast.
Medium
After a few short minutes the beans reach this roast.
Dry
Sweeter than light roast; more body exhibiting more balance in acid, aroma, and complexity. Smoother than the traditional American "medium" roast, but may display fewer of the distinctive taste characteristics of the original coffee.
Medium/Dark, Dark
After a few more minutes the beans begin popping again, and oils rise to the surface. This is called second crack.
Slightly shiny
Somewhat spicy; complexity is traded for heavier body/mouth-feel. Aromas and flavours of roast become clearly evident.
Vienna, French, Italian
After a few more minutes or so the beans begin to smoke. The bean sugars begin to carbonize.
Very oily
Smokey-sweet; light bodied, but quite intense. None of the inherent flavours of the bean are recognizable.