It's a really big particle accelerator, and it's interesting in it's own right. It can speed particles up to 97% the speed of light, so it'll be a boon to physics I suppose.
The thing has also been the focus of people who worry about something going wrong with it. Theoretically, it can create a black hole, but even if it does, there's really not enough mass around for it to be self sustaining. Mostly it's harmless, but still it was a bit amusing to see this article:
http://www.joystiq.com/2008/09/09/terrible-news-gordon-freeman-spotted-near-large-hadron-collider/
If you've never tried to play Half Life you prolly won't get it.