Google Earth is a desktop software program that lets users browse and search the Earth and its topography. The program takes advantage of specially created data layers that can add all sorts of features to the surface of the Earth, including local landmarks, businesses, and any other set of data that have been specially coded to match up with the Earth's geography.
In addition to static layers, Google Earth is also capable of displaying moving sets of data, a feature that has been used to show changes to Earth over time, both long and short. One of the most notable uses of this was to show hurricane Katrina's movement into the Gulf Coast back in 2005.
The most recent version of Google Earth added the capability for users to record themselves using the software to share with others, as well as change what time the satellite imagery was taken to see past landmarks or features.