(Google Earth seems to be a follow-on from Google Maps - using the map information, plus information on locations of landmarks, places of interest, and for selected places, 3D information on landscape and buildings, to give a spectacular view of the world - even easier to use than Google Maps, and that's saying something. Currently, it's a Windows-only app in a very Beta version, but they've stopped allowing downloads now, with the following message :
We appreciate your interest in accessing 3D geospatial information, but due to the technical limitations of this beta launch, Google Earth has been forced to take a brief breather. Please check back with us soon.
I got to see it on someone else's PC at work earlier today; maybe they still have the downloaded file and can send it to me)