It works like this: registered users can create their own map(s) and using so-called “place mark” pinpoint locations, adding comments.  Locations can be chosen manually/graphically, or using Google Maps to locate specific places, through the search function.

A line tool makes it possible to draw routes on your map, and a freeform tool enables the user to demarcate areas.  Google provides the tools in various colors, line types and marker icons.

Google Maps Marker:

If the user wants to add photos or YouTube Videos, they are able to add by uploading them to the customized Google Markers.

The resulting maps can be categorized public or private, a link may be e-mailed or a differently generated link may be embedded in a web-page.

Contrary to the “unpublished” maps, the public ones are accessible to anyone.  They are automatically included in Google Maps and Google Earth search results. More recently, the contents of these markers and maps has been also included in locality-based searches under the generated results, found under the link “User generated content”.
Google Maps User Content

For those out there who aren’t sure why My Maps is a good thing, take a look at the numerous examples of Google “My Maps” creations at Ongopongo.