http://www.boston.com/bigpicture/2008/08/the_large_hadron_collider.html
The CMS is one of two general-purpose LHC experiments designed to explore the physics of the Terascale, the energy region where physicists believe they will find answers to the central questions at the heart of 21st-century particle physics.
I've glanced over the article twice now, and I'm still not exactly sure what this thing actually does, aside from looking totally awesome.

