Pandora is a music discovery and recommendation service. Users can listen to tracks via a simplistic player, and if they like or dislike a song they can vote yes or no on it. If they don't like it, the service will automatically skip it and move onto something else. If they do like it, Pandora will pull up tracks it thinks are similar in style. With enough use, it can effectively introduce you to all sorts of new music, and users can make their own radio stations based on personal tastes.
The service makes its money from advertisements that are inserted every few songs, similar to terrestrial radio stations. There's also a subscription service that lets people skip and re-listen to songs from the playlists they can enjoy ad-free. Besides the Web, Pandora can also be found on mobile phones and home theater devices.