All you do is go to the site, sign up, download a plugin for the media player you use (iTunes, windows media player, winamp, etc...) then the name of every song you listen to on your computer will be sent to Last.fm and added to your profile.
For example, check out mine. I just registed a few minutes ago. My username is YourBackDoorMan
Later on, you can see the top arists, top tracks, stuff like that, as well as recent tracks.
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