From the WWT website from which the program can be downloaded or run on the web:
"From web to desktop to full dome planetarium, WorldWide Telescope (WWT) enables you to explore the universe, bringing together imagery from the best ground and space-based telescopes in the world and combining it with 3D navigation. Experience narrated guided tours from astronomers and educators featuring interesting places in the sky. You can research and import your own data and visualize it, then create a tour to share with others.
A web-based version of WorldWide Telescope is also available. This version enables seamless, guided explorations of the universe from within a web browser on PC and Intel Mac OS X by using the power of Microsoft Silverlight 4.0."
