String theory is a way of reconciling General Relativity with Quantum Mechanics. It is, at the moment, the most popular theory for doing so. You see, Special Relativity works great as a theory for for large objects. But once you get really small, you run into problems. To reconcile Quantum Mechanics and Special Relativity you need a Quantum Field theory. Well there has been some success in this area. There is a Quantum Field Theory for the strong nuclear force, and one for the Weak Nuclear force. But developing quantum field equations for gravity that are consistent with Special Relativity has proved elusive. Having such a field theory would potentially lead to a Unified Field Theory that would be the Holy Grail of the Physical Sciences: a single equation that could describe physical phenomena.
String Theory holds out the promise of this. But there is a problem: at the moment, String Theory is religion, not science. In fact, it can't properly be called a scientific theory at all.
As I've discussed in the evolution "debate", a scientific theory must be falsifiable; that is it must be able to make some predictions that can be tested. This has yet to occur with regards to String Theory. At the moment, it is strictly a mathematical exercise. One of the problems is that the strings are so damn small we don't have any tools that can see them. Of course, the other way to test the theory is to predict the behavior of a system that is consistent with String Theory alone. But to do this, you would need massive amounts of energy.
This is where the Large Haldon Collider comes in.
This is, or will be, a particle accelerator located in Switzerland and being built by CERN. It's so big that it invades France. And with it, scientists hope to get particles accelerated very close to the speed of light and then crash them into each other unleashing huge amounts of energy.
With such a device, some of the predictions of String Theory may be able to be observed. Some of the goals related to String Theory that may be observed by the LHC are
- Is the popular Higgs mechanism for generating elementary particle masses in the Standard Model violated? If not, how many Higgs bosons are there, and what are their masses? 
- Will the more precise measurements of the masses of baryons continue to be mutually consistent within the Standard Model?
- Do particles have supersymmetric ("SUSY") partners?
- Why are there violations of the symmetry between matter and antimatter?
- Are there extra dimensions, as predicted by various models inspired by string theory, and can we "see" them?
Well that's cool, but some physicists fear that the LHC may destroy the Universe.
...while Physicists have the logistics of the LHC well in hand ideas about its outcome are strictly theoretical. According to one scenario tiny black holes could be produced which hopefully would decay into what is known as Hawking radiation (the tinier the black hole, the faster it evaporates). If these black holes fail to decay, however, the consequences could be disasterous. CERN software developer Ran Livneh has expressed some concerns about the project:
"This physical realm is unknown, and dangerous phenomena might arise…Any physicist will tell you that there is no way to prove that generated black holes will decay. The consequences of being mistaken are unfathomable. This subject deserves serious unbiased discussion."
It would probably be bad if a black hole was generated in the middle of France (right?) mostly because it would not be restricted to France. And while "destroy the Universe" might be a bit hyperbolic, some do worry it might destroy our corner of the universe and that's pretty much all that concerns us.
Some other concerns that have been raised include
- Creation of a stable black hole
- Creation of strange matter that is more stable than ordinary matter
- Creation of magnetic monopoles that could catalyze proton decay
- Triggering a transition into a different quantum mechanical vacuum (see False vacuum)
CERN did and investigation and concluded
"We find no basis for any conceivable threat."
Cool. I guess we'll find out who's right in November of 2007 when the LHC is projected to be switched on.