The main Google Maps site is at http://maps.google.com/. The standard user interface includes road maps, satellite images, terrain maps, and traffic reports; although these are not available for all locations. This is what the standard user interface looks like:
Google provide the Google Maps API as a free service for non-commercial use, but reserve the right to display advertising at a future date. The Google Terms and Conditions should be consulted regarding commercial applications. The API allows Google Maps to be embedded in a web site, and requires a key. This key can be freely obtained from Google, but it allows Google to control over-use and request flooding. The API documentation can be found at http://code.google.com/apis/maps/documentation/ .
The following simple example is based on the interactive distance calculator at http://www.mileage-charts.com. This example searches for the start and end locations. If both are found, then the route is calculated and displayed on the map. Google Maps can provide the route directions, although they are not used in this example.
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With active competition from Microsoft's Virtual Earth, Google Maps continues to see regular improvements in functionality, the API, and data coverage. This active competition will continue to strengthen both products for at least the next year or so. This can only be a good thing.