Garmin has a variety of basemaps in their GPS receivers at this time.  Here is the lineup with a few comments.

1)  The III+, 12Map, NavTalk, ColorMap SP & eMap have the exact same base map.
This is referred to as the Americas Highway basemap with exits.  This is the latest version of Garmin's USA base map library.

2)   The StreetPilot has a totally different basemap that is not found in any other unit. The SP basemap is not as detailed as #1 above,  but is a bit more detailed than the G-III(not plus) basemap in #3 below.

3)  The III and the GPSMAP 162 /168 sounder have the same basemap - referred to as the Americas basemap.   An International version of this basemap is also available.

4)  The latest basemap on offer is the Garmin ATLANTIC basemap.  This is now available in the G-III PLUS,  (but not yet available  in the other related products) that has a more detailed basemap of Europe. (See comparisons HERE)

5)  Other units such as the G-12xl,  G-12CX,  G-II PLUS do not have a basemap.  Rather,  they have an extensive "CityPoint Database" which consists of fixed waypoints not under user control for tens of thousands of cities,  towns,  villages.   This database comes in six variants depending on which part of the world you live in  and the data base CANNOT be changed.

Once you have a basemap or CityPoint database installed in a unit it CANNOT BE CHANGED OR UPDATED.

Samples of these various base maps can be viewed by reference to our product reviews on our website.