They have an application you can download. Theres custom radios you can listen to. If you enter artist name(s), the radio will play similar artists.
Also, it keeps track of all the music you listen to in iTunes, windows media player, whatever. Then you can view your top tracks, top artists, and recently played songs. You can then put that stuff on your blog, myspace, site, whatever.
I was thinking about adding last.fm data to the profiles but I figured not enough users here would have an account, and it's a little off-topic.
It's a great site if you like music.