There are many documents available from the Internet which discuss road hierarchy and classification- where one use of “classification” refers to functional classification – for example whether the purpose of the road is to serve as an access road or as a long-distance primary road. However roads can be classed in many other ways as well.
I have worked up a list of different ways in which a road has been “classified”, and posted it as a downloadable, free, five-page document on Scribd (see here). The list in the document is a work in progress. It does not claim to include all the ways in which roads are classified.