Mapsource is older program. It is universal, can support road, topo and marine maps. It is most useful for road maps.

BaseCamp is a dedicated program for topo maps and outdoor GPS. It doesn't support marine maps at all. You can use it for road maps too, but it miss most advanced features of Mapsource, like true addres search, detailed configuration for routing, area and road avoidances. Instead BaseCamp has other features, that Mapsurce is missing, for example support for raster maps or possibility to use map directly from GPS attached to PC.

There is a 3-rd program from Garmin called HomePort. This program is dedicated to marine maps, looks a bit similar like BaseCamp. HomePort is not free.

City Navigator are Garmin route maps. CityXplorer are small parts of City Navigator that Garmin is selling independently. CityXplorer maps have additional data for public transport. This additional data allows for "enhanced pedestrian mode" in nuvi 1xxx, 2xxx and 3xxx. For all other GPS cityXplorer behaves like City Navigator, only with limited map area.