It's a little dense, but this article from Newsweek should help explain it in simpler terms:
In the tunnel, powerful superconducting magnets steer protons around a ring where huge voltages accelerate them until they pick up an amazing amount of energy-7 trillion electron volts at their peak. Over the next few months, as technicians bring the vast machinery online, high-energy protons will be made to collide with one another, causing them to break up into thousands of smaller particles, whose short, violent lives will be recorded by nearby detectors.
The experiments being conducted with the LHC (at CERN, on the French-Swiss border) will attempt to find something called the Higgs boson or "God particle," which no one has even found before (though it is thought to exist). If this sub-atomic particle is found (it's smaller than an atom, and even smaller than a proton), physicists may finally be able to explain how the four forces of our universe -electromagnetism, gravity, and the '91strong' and '91weak' forces - work together to make the whole universe continue to exist. Apparently physicists know how electromagnetism and strong and weak forces work together, but they can't fit gravity into the mix. The aforementioned Newsweek article explain the problem in a nice, easy to understand way (I would cut and paste for you, but you should read it because I can't explain it).
On another note, some have suggested that this thing, this $9 billion dollar thing, may destroy the frickin' world by creating a black hole that sucks Earth in and tears us all into tiny particles of nothingness.
LATE BREAKING UPDATE: World wasn't destroyed. More here including this comforting set of sentences:
A host of Jeremiahs have been predicting that the collider, costing 'a35bn and a quarter of a century in the making, will destroy the world by spawning mini black holes which will sink to the Earth's core before gobbling it up.
The scientists at CERN have dismissed the claim as ill-informed nonsense, and it certainly wasn't going to happen on today's inaugural test run, which did not include the sub-atomic collisions needed to produce an Earth-munching singularity.
But that's not really that likely. And it may put some pieces of the puzzle together that tell us the answer to the ultimate question.
But we already know that the answer is 42. (I had guessed it was 7).
The History Channel aired a special on this thing called "The Next Big Bang" Tuesday night. It re-airs Saturday at 4 pm.