Within the 'My Maps' section of Google Maps, exist a few different types of Maps, all with different features:
This article is only exists now, for historical reference
1) User Editable
These are the standard maps created whe click 'create new map' these are a store of features defined by the user, the actual features are stored on Google servers and can continue to be edited in the future by the map owner. In addiction the owner can elect to allow others to collaborate on the map. There is a hard limit of 1000 features per map, and between 60-200 features are shown per 'page'
These offer import and export features, via GeoRSS or KML, and can be viewed in Google Earth. Embedding single maps is easily done directly via the Maps website.
These exist simply as a link to display content loaded dynamically from elsewhere. For example pasting a KML or GeoRSS url into the search box, upon clicking 'Save to My Maps' its saved as a geofeed to the 'created by others' section of My Maps. (geofeeds are actully implemented as a very specialist mapplet, that just records the source data URL)
(these can be 'embedded' on other websites, if click 'Link' before adding to My Maps)
(these can not be embedded on other websites, but some might be suitable for conversion to the full Google Maps API)
Todo: add links and and elaborate on some of the features mentioned above. If want clarification, add a comment below...