The LHC (Large Hadron Collider) project began to be thought up way back in the 1980's. Though just these past couple of years have seen the first experiments conducted in the 27-kilometre long tube, the intellectual beginnings date back 2.5 decades.
When fully functional, the LHC will accelerate a stream of particles to 99.99% the speed of light in one direction around the 27km long tube, and do the same to another stream of particles in the other direction. When at full speed, the two stream will be made to intersect each other, and the particles will collide. What happens after that is what scientists are anxious to find out.
The Large Hadron Collider (The Hadrons are particles, the collider points to the fact that it collides particles, and the large is...well, 'cause it's large) is the largest and highest energy particle accelerator ever built on Earth. Physicists hope that the LHC will answer fundamental questions about the relationship between quantum mechanics and general relativity, among other questions. One of the most important expected results from the LHC experiments is finding out if the Higgs-Boson particle will show up or not. Whether it does or doesn't, the outcome will likely change many particle physicists' theories forever. If it does show up, it will complete the Standard Model, what many scientists think is the most likely theory concerning the electromagnetic, weak and strong nuclear interactions which mediate the dynamics of the known subatomic particles. If it doesn't, then according to Michio Kaku, a famous leading physicist ranked second only after Stephen Hawking, the Standard Model will be refuted and scientists will have to concede that what they thought about the way subatomic particles worked was completely wrong. Kaku finds the latter outcome just as exciting as the former, if not more.
Initially, there were fears that the machine could create tiny black holes that could grow to indescribable sizes, consuming the entire earth, but LHC officials say that if black holes are created, they will collapse and disappear almost instantly, and cause absolutely no damage.