It's all starting again! (Last summer, many questions were asked about this subject.)
The experiement is called the Large Hadron Collider (LHC).
It's large, because it's underground a very large area near Geneva. It's called the hadron collider because it takes a load of particles (of a type called hadrons), and throws them at more hadrons.
The result of this, if it's done at a high enough energy, is that new particles are produced in all the mess. Much like throwing two bricks together at great speed, you get a load of mess, and some (perhaps) interesting small stuff you might have missed othewise.
There is a standard model of particles that we use to predict new particles (kind of like the periodic table; just a way of finding some order). We call this the standard model, because some people just have no imagination.
One problem with this is that it doesn't suggest where particles get their mass from. One suggestion some years ago was that it was down to another, really hard to find particle, called the higgs boson. So this is just one of the things they're looking for.
It's using the same tunnel system that has been in place for some time now, just new detectors and other systems inside and around the tunnel.