The premise is simple -- by constantly pinging all of 'the best' music blogs The Hype Machine creates a list of the music that is being discussed at any given time and makes it all available to listen to. The idea is that you'll see some new track/artist listed on the main page that you like, sample the music (streaming, no downloads) and then choose to buy the song (iTunes, Amazon, etc) thus giving The Hype Machine an affiliate's cut of the sale. Of course they also link back to the blog that listed the track in the first place, thus hopefully generating greater traffic for the originating blog, too.
In the top right corner of the page is a Search box. Typing in an artist brings up all the hits in the database, all of which can be streamed in a nice simple flash player.
If you like pandora I'm willing to bet you'll find something to like about the Hype Machine, too. For people who want to stay abreast of the latest and greated in the music world, I see this as being an invaluable tool.