The LHC at Cern will be switched on today, though there won't be collisions for a little while. The LHC will collide Hadrons (obviously) at high speeds. Hadrons are particles which contain quarks. Everyday examples are Protons and Neutrons. The LHC can only accelerate charged particles and so it'll collide protons. The high energy collisions will produce new and interesting particles - and the hope if to find the predicted Higgs Boson. Though if it's not found, that'll also be interesting....
There has been idle chat that it could create miniature black holes that'll destroy the Earth. This will not happen. It probably won't create black holes. But if it does create black holes they'll be moving so fast that they'll leave the Earth. Even if they don't they'll evaporate due to Hawking radiation as they're so small... and in the unlikely event that it does create a black hole that hangs around to destroy the planet - there'll be nobody to contradict me.
Seriously: cosmic rays have collided with the Earth with huge energies since time immemorial - a particle accelerator experiment in the upper atmosphere. We're still here.
(P.S. I'm not saying this to boast, only to show that I know what I am talking about: I post as someone with a damned good Physics degree. Irrelevantly, I've visited LEP, the Large Electron Positron Collider and LHC's predecessor. Tragically, I took time out of my first USA trip to go to Fermilab. I've been to RAL in the UK many times.)