Does the Large Hadron Collider have the potential to unlock the secrets of the universe? Absolutely! Will it bring us fantastic new technologies and revolutionise science? Definitely! Will it destroy the Earth? No way.
The thing is, part of the problem is that what actually happens in the experiment is not well understood by the average member of the public. That's quite alright; I study physics and it still amazes me how it all works. At its most simple level, the LHC gets tiny particles of matter (the "hadrons") and smashes them together (hence "collider") at very high speeds. To do this, it needs to be pretty powerful (stuck for synonyms, someone decided to call it "large").
Before we can understand how it works, though, we have to know a little bit about what its looking for. Matter, or "stuff", is made up from tiny particles called atoms. Atoms are, in turn, made up from even tinier particles called protons, neutrons and electrons. Protons and neutrons stick together in roughly equal numbers and make a dense nucleus or "centre", whilst electrons orbit this nucleus at high speed.
According to current theories, there should be a particle called the Higgs Boson which is effectively responsible for objects having mass. The trouble is, no one has found one yet. One way to look for one is to build a giant detector and sit around waiting for a Higgs Boson to fly into it. That's quite unlikely though, since they don't last long and it's very hard to predict when and where one might exist.
A better way of spotting this Higgs Boson, then, is to actually make one. By smashing protons together at extremely high speed, they release energy and make a shower of particles, one of which might be a Higgs. If the detector is looking at the right place at the right time, it should see it. That, in a nutshell, is the LHC's goal. It also has many other experiments to run using the same device, because really, the LHC is just a big circle of magnets that are used to speed up particles. That means that lots of different types of particle can be smashed together, and each collision can provide detailed information that helps further scientists' knowledge of the universe. The results could benefit technology more than any other experiment in human history - just imagine, one day we might even be able to manipulate the very building blocks of the universe itself.
Some people have been worried that these high-energy collisions might threaten our planet by causing black holes. Technically speaking, a black hole is an object that has so much gravity not even light can escape, and are usually rather large (somewhere in the region of 10 times the mass of the sun. The LHC is capable of producing micro black holes, which despite the name are actually very different in their behaviour. A micro black hole is simply too small to have any effect on the rest of the world and will evaporate almost instantly.
Fortunately, this means the risk of destroying the Earth is practically non-existent. In fact, although the collisions sound quite powerful, stronger collisions happen between the Earth's atmosphere and particles from the sun all the time and have been doing so for billions of years with no ill effects. The overwhelming majority of the scientific community agree that the LHC is perfectly safe and the potential benefits greatly outweigh any risks.
Also, there's a rather big reason why the LHC isn't going to end the world any time soon it's broken.