A recipe. Build two pipes, each about 6 centimetres wide and 27 kilometres long. Bend them both into a circle and cool to 1.9 kelvin, about 300 °C below room temperature. Fill with protons - about a hundred billion of them to start with - all travelling as close to the speed of light as possible. Add a magnetic field a hundred thousand times more powerful than the Earth's to steer the particles through the pipes. And make sure the protons in each pipe move in opposite directions.Wired too has an insightful take on the Best and Worst Case Scenarios for the world's costliest scientific experiment. See also the funny FAQ in the sidebar.
Now here's the exciting bit. Align the two pipes so that the particles collide. About twenty protons should smash into each other, creating showers of other particles. Take a good look at each one. If you spot a particle you don't recognise, shout.
Finally, repeat forty million times. Each second.
- New Scientist 1994 article
UPDATE: Here's an even better explanation in a CERN rap video.