If you're new to KML, begin by browsing the KML Tutorial, which presents short samples of KML code that you can view in Google Earth and Google Maps.
The KML Reference provides detailed syntax for all KML elements, with explanations and diagrams of how to specify them.
The Developer's Guide contains in-depth conceptual material and examples.
Creating and Sharing KML Files
You can create KML files with the Google Earth user interface, or you can use an XML or simple text editor to enter "raw" KML from scratch. KML files and their related images (if any) can be compressed using the ZIP format into KMZ archives. To share your KML and KMZ files, you can e-mail them, host them locally for sharing within a private internet, or host them publicly on a web server. Just as web browsers display HTML files, Earth browsers such as Google Earth display KML files. Once you've properly configured your server and shared the URL (address) of your KML files, anyone who's installed Google Earth can view the KML files hosted on your public web server.
Many applications display KML, including Google Earth, Google Maps, Google Maps for mobile, NASA WorldWind, ESRI ArcGIS Explorer, Adobe PhotoShop, AutoCAD, and Yahoo! Pipes.