Google Earth is a geography-program that is made by Google. It was originally made by Keyhole Inc. and called Earth Viewer, but in 2004 Google aquired it and changed the name to Google Earth. Here you view the entire planet from above using a mix of sattelite images, cleverly put together to cover the whole globe. You can find your way in cities, and search for all the landmarks you know about. There are even web pages that covers weird things that people have found. Maybe a car crash happened at the exact time the satelitte picture was taken, or maybe you find a burning building. Google Earth requires a internet connection, as most of the satellite images are downloaded dynamicaly when you are using the program.
The quality of the satelite images varies, som of the heaviest populated areas have good enough quality for you to see people, while the less populated areas have much lower quality. But the images are continuously being updated and improved. You may also download different plug-ins to make the viewing more interesting. Or you could download houses or cities in 3D, and if you are interested, you can model houses that you are familiar with using Google Sketchup and put them up for others to download.
Google Earth can be really captivating. You run the risk of having your whole day fly by you, as you sit and look for strange aircrafts parked in Area 51, or scan the lawn outside the White House for the president. But it is both fun and educational to use, and many places have links to pictures or articles in Wikipedia.