"KML is a file format used to display geographic data in an Earth browser, such as Google Earth, Google Maps, and Google Maps for mobile. KML uses a tag-based structure with nested elements and attributes and is based on the XML standard."
You can create a KML file in Google Earth: add placemarks, image overlays, paths, and polygons, then save the file. You can also convert a CSV file to KML, a RSS file to KML, or a spreadsheet.
What can you do? Aggregate the 10 most expensive homes in the US, annotate the most important monuments from the Ancient Rome, share where you have traveled recently or your company's presence around the world.
Until now, some of the most interesting KML files could be found in different places, like Google Earth Hacks. You could also use the filetype: operator in Google search to find a KML file.
But now KML search is integrated into Google Earth:
Type a location and then search for something special around that area. For example, search for "Italy" and then for "volcano" and scroll down to the KML results. Then click a green sphere and the link from the bubble that opens, to see the KML file.
Chikai Ohazama, product manager at Google Earth says: "Users can now search through all of the world's Keyhole Markup Language (KML) files, making the millions of Google Earth layers on the Web instantly accessible for geobrowsing and exploration." He also promises that KML search will be available in Google Maps too.