Dark roasts are dark brown, sometimes almost black, in color. They resemble chocolate, if it was shaped like a coffee bean. Oils can be seen on the beans at this point.
When drinking a dark roast, you’re almost exclusively tasting notes from the roast. The brightness of light roasts is replaced with body in dark roasts.
Historically, dark roasts have been popular in Europe, giving rise to terms such as Continental, Italian, French, and Spanish roasts. Espresso roasts are also usually dark roasts, which is partly why espresso can stand up to lots of milk and sugar.
Roast level is largely a personal preference, as each level produces different qualities in the coffee. Knowing whether you prefer light, medium or dark roasts, though, can help you identify new coffees that you might like.