What is Google Sky?

Google Sky is a part of the Google Earth application and allows users to omnisciently explore the universe with a computer connected to the internet. It's like a personal planetarium. Google Sky, which was announced on 22 Aug 2007, now contains several images of the radio sky made using NRAO instruments.

What is Google Earth?

Google Earth is a cross-platform desktop application which allows users via internet to see virtually any place on earth, drawing from sources of satellite images, maps, terrain data, and 3D models. It now includes a sky view to allow users to explore the universe in similar fashion; this is Google Sky.

Formerly known as Earth Viewer, the product became part of the Google family in 2004 after Google acquired Keyhole, Inc. in 2004, renaming it to Google Earth in 2005.

A history from Wikipedia (as of 20 Dec 2007):

"In version 4.2, released August 22, 2007, Google Earth added a Sky tool for viewing stars and astronomical images.^[9]Google Sky is produced by Google through a partnership with the Space Telescope Science Institute in Baltimore, the science operations center for Hubble. Dr. Alberto Conti and his co-developer Dr. Carol Christian of the Space Telescope Science Institute, plan to add the public images from 2007,^[10] as well as color images of all of the archived data from Hubble's Advanced Camera for Surveys. Newly released Hubble pictures will be added to the Google Sky program as soon as they are issued. New features such as multi-wavelength data, positions of major satellites and their orbits as well as educational resources will be provided to the Google Earth community and also through Christian and Conti's website for Sky. Also visible on Sky mode are constellations, stars, galaxies and animations depicting the planets in their orbits. A real-time Google Sky mashup of recent astronomical transients, using the VOEvent protocol, is being provided by the VOEventNet collaboration."