pandora.com

that will explain it all.
what it does is it analyses the features of the music you currently like, by typing in a band, or song name, then it will pick a song that is similar to that, play that, it is often the same artist or song as you first picked. Then you rate how you like the song, good or bad, then it automatically picks the next song it feels you would like. I strongly suggest finding your favorite song and using that, it works best that way.

it will find songs and bands you never heard of, or have never thought to listen to. And its free.