Last.fm streams music to your MP3 player. While each song is playing, you select whether you want to listen to 'similar' or 'different' music. Last.fm remembers what you listen to in your profile (your record collection), and decides what music to play you by comparing your record collection with other users. The better you fill your record collection, the better music Last.fm will play to you and to everybody else.
(Make sure you allow it to open the player pop-up window when you sign on, otherwise nothing will happen).