Here's the blurb from their FAQ:
Pandora is a music discovery service designed to help you find and enjoy music that you'll love. It's powered by the Music Genome Project, the most comprehensive analysis of music ever undertaken. Just tell us one of your favorite songs or artists and we'll launch a streaming station to explore that part of the music universe.I've been having fun throwing some fun tunes at it. "Blow Me Away" from Breaking Benjamin (Halo 2 Soundtrack), "Turn the Page" from Metallica, and even "The Current" from Blue Man Group. Got some interesting tunes to listen to. First 10 hours our free, then there is a subscription fee.