The official post is simple:
create, share, download, and listen to thousands of NIN remixes in this new community of user-generated creativity. http://remix.nin.com
But the site is very rich, and well done. The concept is that Trent Reznor is making the original multi-track files available for anybody to change, add to, remix, destroy, and then upload it to the site for everybody to hear. There is a social network aspect (voting for uploaded tracks, commenting, etc) and every thing can be downloaded as free mp3 files.
The entire site is also RSS enabled- so you can subscribe to news feeds, or track listing feeds as well as subscribe to category feeds (or individual artists) for remixes which will download as podcasts of the actual mp3s.
When I heard that NIN was doing this, I was very excited at the thought, but the execution is just amazing!