Keyhole Markup Language (KML) is an XML-based language built by Google™ and is mainly used in its desktop mapping product, Google Earth™. It allows the user to virtually view, pan, and fly around the imagery of the Earth.
Playing with imagery can be really nice; if you haven't yet, you should try just wandering on remote areas of the planet and explore areas where once the Here be Dragons label were the most common among labels: (http://en.wikipedia.org/wiki/Here_be_dragons).
However, the program is way more than a videogame and if you add it to your data, it becomes a very powerful and accessible tool for 3D visualization.
How do you make GeoServer and Google Earth™ interoperate? It is here that KML comes to your ...