Keyhole Markup Language (KML) is an XML markup language used in SketchUp, Earth and 3D Warehouse. It is an open format that can be used in many applications beyond Google. It comes in the KML and KMZ (Zipped version) formats.
KML is an XML format that describes the integration of mixed media into an application like Earth. KMZ is a container file for KML and its media objects. Inside a KMZ file, a 3D model is in the dae format.
KML and KMZ files are easy to create by hand. But of course SketchUp is needed for real 3D modeling.
Export KML/KMZ files from SketchUp from the menus "File > Export > 3D Model" and then choose file type "Google Earth KMZ".
Share models on the 3D Warehouse from the menus "File > 3D Warehouse > Share Model".
An example KMZ file, SID.kmz contains
doc.kml models models/SID.dae images images/texture0.jpg images/texture1.jpg images/texture2.jpg images/texture3.jpg images/texture4.jpg images/texture5.jpg images/texture6.jpg images/texture7.jpg images/texture8.jpg textures.txt
KML for Google Maps Edit
Dynamic Live Data Edit
<?xml version="1.0" encoding="UTF-8"?> <kml xmlns="http://earth.google.com/kml/2.0"> <NetworkLinkControl> <minRefreshPeriod>60</minRefreshPeriod> </NetworkLinkControl> <Document> <name>USGS Real-time Earthquakes</name> ... <NetworkLink> <name>Earthquake Epicenters</name> <open>0</open> <Url> <href>http://earthquake.usgs.gov/eqcenter/catalogs/eqs7day-age_src.kmz</href> <refreshMode>onInterval</refreshMode> <refreshInterval>300</refreshInterval> </Url> </NetworkLink> ... </Document>
The NetworkLink element references the source of this file (itself).
Incremental updates Edit
The Regions feature allows streaming large datasets for a smooth view. Viewing can begin before the entire file is completely downloaded. Level of detail improves as the streaming download continues.
See this note from KML Support for caveats.