Keyhole Markup Language. Its what makes Google Earth cool and exciting.
Most often its found in KMZ (or zipped) versions.
You can open it up in a text editor and its mainly just html and xml.
<?xml version="1.0" encoding="UTF-8"?> <kml xmlns="http://www.opengis.net/kml/2.2"> <Document> <Placemark> <name>New York City</name> <description>New York City</description> <Point> <coordinates>-74.006393,40.714172,0</coordinates> </Point> </Placemark> </Document> </kml>