Congratulations on the Roadmate. I haven't used one, but most of these portable GPS units work essentially the same way. They contain electronic versions of maps and, with the use of the GPS antenna/receiver, locate your position on the map and continue to track your position as you move. How you get directions will vary a little between models, but again is essentially the same. e.g. you input an address or final destination, let the unit find it, then tell it you want to go there. The directions will be provided both visiually and, if you choose, audibly.
Two differences between a portable GPS and using Mapquest online are that Mapquest can't track your position as you move and it requires an internet connection.
Hope I understood your question correctly.