Pandora RadioPandora Radio is a website, (There are also apps for various types of phones) that is based around recommending music for users based on music and artists that they like.
What you do, is create a "radio station" by selecting a "seed", which is basically just a song or artist that you like. Pandora Radio will then create the station based on that and play a song by that artist. Each song on Pandora Radio has been dissected by a team of people and categorized by elements in each song. After playing the first song, Pandora Radio will play another song, by a different artist, but a song that has similar elements to the original. The plan is that you will like this new song.
Now, if you do like this new song, you can give the song a thumbs up, this means that you like the song, and this song will now be used along with your original seed to select new songs. You can do this for every song that plays. If you don't like the song, you can give it a thumbs down, this tells the system that you don't like the song, and it will not be used to select songs. Also, you can tell Pandora Radio not to play a song ever again. Alternatively, you can not give any songs a thumbs up or down and all new songs will be based solely on the original seed.
As far as I know, you can make as many stations as you wish. Also, you can add seeds to pre-existing stations to further change what new songs are selected.
And it's not just standard music that is on Pandora Radio, there is also video game and movie music. For example, I created a station with my seed as Nobuo Uematsu, and the first song that played was an orchestral medley of Final Fantasy III music. I created another station, with Yasunori Mistuda as a seed, and Pandora Radio proceeded to play music from Chrono Trigger. Another station I created used Harry Gregson-Williams as a seed, and the first song that played was the Metal Gear Solid Theme Song. A while later, music from The Chronicles of Narnia was playing on the same station.
The team behind Pandora is constantly adding new music, and adding elements to songs to help Pandora better match music to what you like.