MapSource is the mapping application for communicating with Garmin GPS units. It includes the same base map that comes with your GPS. For my Legend, that was interstates and most state roads. You can create routes and waypoints and download them to your GPS. You can also pull track logs and waypoints off your GPS.
I keep a base set of waypoints on my GPS for places such as "home" and other common destinations. With the waypoints on the GPS, I can quickly calculate a route "home". For longer routes, such as a century ride, I might also enter waypoints for rest stops.
Detailed road maps, which are anything more than interstates and state roads, are a separate purchase. The routable maps for my Legend are called City Select North America. With these maps, I can use MapSource to plan routes and download those routes to my GPS. I keep lots of routes on my PC.
I generally keep my MapSource data stored in the propriety Garmin format, gdb. If I need to access the data from another application, I'll export the route as a gpx file. For example, if I want to ride up Big Cottonwood canyon and know what kind of elevation profile I'm getting into, I'll map a route up the canyon with waypoints spaced about equally apart and with mostly straight road in between them. Then I'll export that as a gpx file. I'll load that gpx file in TopoFusion and using some functions it has, which I explain on the TopoFusion page, I'll get a fairly precise climbing profile up the mountain.
I've seen MapSource take some pretty heavy criticism in some online forums. I think it's the best application for making calculatable routes on Garmin GPS units. I also think it's not too bad for waypoint management. I never use it to download track logs from my GPS.