Google Maps Implementation
Google Maps uses JavaScript to power much of the implementation on the client's web browser.
When you drag a Google map, the grid squares are downloaded from the server and inserted into the page on your web browser. If you search for a business, the results are downloaded into the side panel and map without reloading the page. Locations are indicated through dynamic rendering of a red pin composed of several partially-transparent PNGs on top of the map images.
Google Maps uses JSON for data transfer rather than XML, for performance reasons, under the umbrealla of the Ajax project.
Because Google Maps uses mostly JavaScript and XML, others have been able to easily add features to the Google Maps interface. These might include things like custom location icons, flashing coordinates, or foreign language signposts. Tools like Greasemonkey provides many scripts for customizing Google Maps information.
References & Further Reading
Google Maps: General References
Google Maps UK