From the bigger to the smaller: A for Autoroute (E in European standard), N for Nationale, D for Departementale, R for Route Communale (but if you're on such a R road, you're probably already lost ;-).
D roads can be highway wide or near asphaltless tight ;-)
As for fun, in the South West quarter of France, you can't really go wrong on 'D' roads ;-)