Adding to Roddy's answer, the principle behind memory map mapping and Garmin's vector based mapping is totally different so no Garmin device works with Memory Map.
A route in Memory Map is simply a line draw on a computer generated drawing connecting predefined waypoints. No different to marking waypoints on a paper map and connecting them with pencil lines. Memory Map only knows the co-ordinates of each dot, it has no knowledge of the information shown on the underlying drawing/map so cannot use the map information to calculate a route. Garmin vector mapping is different. The level of information shown is a lot less, i.e. just roads, but the software can plot courses along those roads.
There are however standards for defining waypoints and routes which can be understood by products that support either type of mapping.
I define backpacking routes in Memory Map and save as GPX. That makes them loadable into Mapsource and they can be used with my Garmin 60 and the crap Topo mapping.
They could just as easily be downloaded into my Zumo but unless every waypoint was on a road the Zumo could not calculate a route. One could turn off routing so the Zumo would just show the route as straight lines from one point to the next, as with the Garmin 60 or my old Garmin 12, but this would have very limited use.