An "overlay" is really nothing more than method in which you may combine markers from multiple maps to create a new map.  This concept  originated from the need I had to be able to show maps of subdivisions and their relationship to schools.  So, rather than having all on one map, I decided to create two maps ... one for schools and then one for subdivisions.  This enabled me to keep the markers logically separated.  So on content when I needed to show both, I would create a third map that had nothing more than two overlay maps associated to it.  

Here is another example that hopefully spells out the concept a little better. 

Map #1 contains the following markers:
* Marker A
* Marker B

Map #2 contains the following markers:
* Marker C
* Marker D
* Marker E

Map # 3 contains the following markers and overlay:
* Marker F
* Map # 2 (OVERLAY)

The result is that when Map # 3 is rendered, markers C, D, E and F will all be rendered. In this example, markers C,D and E are inherited from Map # 2 which was defined as the overlay. Make sense?

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