It integrates Google's new satellite/aerial database (the one they're using for Google Maps), but includes medium (not low!)-res satellite coverage of the whole earth - not just the US - and has detailed aerial coverage for many more foreign cities (including vector-map layers for some).  

It allows users to create and share annotations that include icons, text labels, lines of adjustable width and color, and outlined and/or filled polygons of variable color/transparency that can be "extruded" upward in space to form column-like 3-D buildings.  

Users can also overlay other maps and graphics onto the 3-D terrain, (including web-based sources that update like weather satellites and radar).  Overlays have variable transparency, so you you can fade them in and out and see both the map and the aerial/satellite data at the same time.  Overlays can also be shared with other users.  

The vector-map layers, which can include street maps, political boundaries, zip codes, park boundaries, local and commuter rail transit, as well as many categories of labelled icons - both yellow-page-like business classes and civic elements like schools, fires stations, churches, and hospitals.  

They've also added the Local Search and Driving Directions feaures from Google Maps (including a cool mode that "flies" along the marked route).  

In addition, they've added a data layer that has pre-built 3-D extrusions of 39 high-rise urban cores in the US.  You can virtually "fly through" the skyscraper canyons and create 3-D perspectives that include the built-up skyline (renedered as grey 3-D colums).  

You can even render the 3-D buildings over the vector maps over your semitransparent overlay over the aerial/satellite photos.  

It's pretty seriously cool. :-)

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