Pandora is a music delivery system that knows a lot about what its users like or dislike about music. It streams songs and users get to thumbs up or thumbs down those songs. With that, Pandora is able to get you closer and closer to an ideal stream of songs that you are likely to enjoy based on the way others like or dislike the music that you do. If you like a song, then Pandora knows what other songs people who like that song like. If you then dislike that choice, Pandora uses that to determine what to play next.
Pandora comes on the web, but also has some good mobile apps. The iPhone app works well and the iPad app for Pandora is even better, with more information displayed about the group that is playing and the chance to easily bookmark information about what is playing (artist or song). Later, from the website, one can see one's profile bookmarks.
Another thing about Pandora is its Facebook integration. If you opt in, you can see what your friends like: their channel subscriptions - or be told that a song is liked by a friend.
If you pay a subscription fee, you don't have to see ads. While there is a forward button (a skip this song button), there is no back button. Also, you can't skip too many songs in a short time. If you try, you'll get a message saying that to skip too many is a violation of their licensing agreements with the music companies. It's a radio station more than collection of music that you control.