Google Earth is the originator of the popular Keyhole Markup Language (KML).
Google Earth started out in 2001 with the founding of Keyhole, Inc. Keyhole were a software development company that specialized in geospatial data visualization, and were responsible for a number of fly-over animations for the news networks. Google acquired Keyhole in 2004, and their marquee Earth Viewer was released as Google Earth in 2005.
Today, Google Earth works as a good tool for plotting data and annotation on aerial photographs or even the entire globe. This annotation is typically using KML. KML has since become a de facto standard for online mapping applications, although only Google Earth supports the most obscure tags such as camera positioning.
The user interface is very easy to use and allows easy navigation of the globe. Image resolutions are very impressive for many urban areas. As an example, here is the current (November 2008) image of the new Dallas Cowboy Stadium in Arlington, Texas.
Resolution of other parts of the globe are variable. On a recent project I found the resolution of northern Costa Rica was in the tens of metres and featured a lot of cloud.
As well as viewing the globe, Google Earth has a 'planetarium' mode, allowing the annotated heavens to be viewed. Objects of interest include their own photos, allowing nebula,etc to be viewed. This might seem to be a bit of a gimmick, but it could be useful for educational applications.
Overall, Google Earth is a good tool for viewing KML map data. The user interface is polished. Although data is being delivered over the Internet, the overall experience is smoother than similar web-based tools. If you wish to create a customized application with KML and satellite data, then Google Maps or Microsoft Virtual Earth should be considered instead.