So, what are these KML and KMZ files anyway? They are Google Earth’s file format for storing placemarks, network link information, and much more. I won’t go into detail here, since there are excellent reference documents available from Google (see below). But, let me describe the fundamentals.
KML stands for Keyhole Markup Language (Keyhole was the name of the application before Google bought it and added their own features and larger databases). If you understand HTML/XML you will have little problem understanding the syntax of KML. You can learn about KML from the KML Tutorial, or get the full details from the KML Documentation.
KMZ stands for KML-Zipped. It is the default format for KML because it is a compressed version of the file. One of the more powerful features of KMZ is that it allows any images you use – say custom icons, or images in your descriptions – to be zipped up within the KMZ file. That way you can share these details without having to reference the files through some link to the Internet. For KMZ files without images, the file size will be much smaller than the equivalent KML file.
If you understand basic HTML/XML you can easily get started by saving a simple placemark you’ve created as a KML file (not the default KMZ), and looking at the resulting text file. If you are a programmer you will probably be fascinated by the topic of Network Links. Network links are one of the most powerful features of GE enabling a KML file to reference data dynamically on a server somewhere out on the Internet.
In a nutshell, learning to do things with KML/KMZ is the equivalent to learning to write a web page, but instead of a web page you are changing the face of the GE. Just like with web page creation tools, you don’t have to know KML to create a KML file. You can start placing Placemarks or Image overlays straight from GE’s interface.