KML is an XML-based language schema for expressing geographic annotation and visualization on existing or future Web-based, two-dimensional maps and three-dimensional Earth browsers. KML was developed for use with Google Earth, which was originally named Keyhole Earth Viewer.
Because of the easy way to create KML data it can be used or generated by numerous applications and not only by google earth. However, there are some very very nice KML files you can download which give you all kinds of insights on for example data of the curren state of the rain forest and the woods around the world.
The Google Earth outreach website is giving you a couple of those KML files for you to enjoy or to educate.
In my opinion mastering KML is something you can benefit from in the near future because more and more applications will be using geo-xxxx data.