One of the questions that I'm often asked when running the Google Earth courses is to talk about the sources of information that can be used in Google Earth.
There are lots of these of course.
Google Earth files are usually stored in KML or KMZ format. The two types of document are quite similar - when several files are zipped up they generally have the 'Z' ending.
Opening up large files will slow down Google Earth and may increase the chances of it crashing - it's not 100% stable on some of the machines that I've used it on.
Here's an example of the sort of site that you might come across: the NASA EARTH OBSERVATORY website.
This has a range of KML files which can be viewed in Google Earth.