Sorry if this is a repost......but this is AWESOME.
What you do is go to the site, type in the name of a band that you really enjoy their STYLE of music, and pandora will create a "radio station" for you that plays music from that artist, and other artists that have a similar sound! After a few songs it requires you to register (it's free, it will include ads at some point, although I've been registered for several weeks now and haven't heard a single ad), and then it remembers all of your radio stations.
It's intelligent too, you can tell it if you like a song, or dont' like a song, and it learns and caters to your tastes!
The only "bad" things about it are that you can't go re-wind or go back and listen to a song over again. They say their music licences don't allow that. Also, if you "fast forward" past a song, you can only do it so many times per hour.. same thing, the music licences.
The main purpose is to introduce you to new music that you'll probably like! It's pretty sweet! Enjoy!
The database is still somewhat limited.......but they are in the process of adding songs to it. Oh, and for those of you at work who have streaming media blockers (like me

).....it uses a flash player so it'll still work!