The Compact Muon Solenoid experiment studies the Higgs Boson and other standard model particles. The CMS collaboration also searches for new particles, e.g. particles that make up dark matter, and probes for the existence of extra spatial dimensions and supersymmetry. All this is done using a giant general-purpose particle detector at CERN's Large Hadron Collider.
Check out this animation to see how CMS was built:
LHC Page 1 - live accelerator status