Pandora is also the name of an internet radio service utilizing the Music Genome Project. The service is free to all users because of the revenue generated from ad space on the site.
The Music Genome Project was started on January 6, 2000 and has the goal of classifying all songs using over four hundred different attributes referred to as genes. These attributes range from "a smooth male lead vocalist" to "major key tonality" to even "a prominent harmonica part". Each song is defined by anywhere from 150 genes for rock songs to 500 genes for classical pieces which are then plotted in space to create a 'vector' for that song.
Like traditional radio, Pandora plays music by organized stations. To create a station, users input a song title or the name of the artist. Pandora then plays songs 'nearby' to the vector which defines that input by using a distance function. Users are then given the option of giving a song a thumbs up or a thumbs down which influences which songs are played on a station. If a song is given a thumbs down, Pandora skips the rest of the song and plays fewer songs with genes shared with that song. Likewise, if a song is given a thumbs up, Pandora selects more songs which share genes with it. If a station becomes too repetitive or selective in its music, you also have the option of adding another artist to the station.
Personally, I love Pandora. I have stations with music from Samuel Barber, Johnny Cash, Frank Sinatra, and Led Zeppelin as well has half a dozen others. The great thing about Pandora is its fusion of the information age with old school radio: it plays music in a random order that you may not have heard of before and introduces you to new sounds free of charge. If you like a song enough, you even have the option of buying it via several online services like Amazon.com or the iTunes Music Store. So it you're ready to put away the antenna and the vinyl and god forbid the 8-tracks, I'd say that Pandora is the way to go.