Basemaps usually contain less information than detail maps and in most cases are built into the receiver. Detail maps are usually loaded into a receiver's memory from a personal computer. (Although they might come pre-loaded on some receiver models.)
Cities & Towns
In basemaps, cities and towns are often marked by a single point near their center or downtown. Detail maps will often show the exact boundaries, allowing you to see the city's shape.
Roads & Highways
Basemaps usually only contain interstates, highways, and a few major thoroughfares. Detail maps will commonly show all local roads and sometimes even include private roads.
Basemaps usually contain mostly information about roads, towns, and cities. Detail maps, however, might also contain information about businesses, shopping locations, topographical data showing contour of the land, or even fishing and navigational information for rivers, lakes, and oceans.
Filling in Areas Between Detail Maps
On receivers with limited memory, you sometimes might load detail maps for areas that are far apart (e.g., your home city and an area you are going to visit). A basemap helps by filling in the areas between detail maps. This works pretty well, for example, as you drive on highways or interstates between two cities where detail maps are loaded.
Zoom / Level of Detail
As you zoom in from showing a map of a wide area, most receivers switch from using their basemap to showing information from a detail map (if one has been loaded for the area). Some receivers have an indicator on their display that shows which type of map is being used.