I got this in one of the other groups, now I finally think I understand.  This is one helluva machine!  I can see why all the controversy surrounds it now.
I'll share it in case there are others who don't quite understand how this thing works.
Suzen

Suzen,

It appears the original question of explaining what the Hedron Collider (LHC) is for and what it does in laymen's terms hasn't been answered yet so I'll make an attempt for you since I have seen a particle accelerator under construction and as a result, I am familiar with what it's used for.  

Basically Suzen, it's a large tunnel structure underground, shaped in a race track (the tunnel allows for maintenance to the actual internal components of the particle accelerator.  Inside the tunnel is a large pipe system, also shaped like a race track (there's a little more to this but for this explanation, you don't really need to know the specifics).  The pipe system is referred to as a particle accelerator.  What that means is that it is capable of shooting atomic particles (atomic particles are like electrons/protons/molecules/etc) at speeds that are near the speed of light (which, of course, is VERY fast).  They shoot these particles in opposite directions in hopes of colliding them together.  The collision is what they are studying in their experiments.  

The LHC was designed for specific experiments like proving the existence of dark matter and help to prove the "Big Bang" theory.  Just in case you're wondering, dark matter is theorized to make up the majority of universe.  When you think of space, you're actually thinking about dark matter since that is space.  At any rate - that's what the LHC is - a particle accelerator.  If I'm not mistaken, particle accelerators allowed for the discovery of very small particles known as quarks, among others (see quantum physics for more on this).  By the way, the size of the tunnel will determine the limits of the types of experiments that can be performed.  The LHC is the world's largest particle accelerator.  

Opponents of the LHC state that because of it's size, the collisions that will occur could be large enough to create minature black holes.  Black holes are holes in space that suck in everything around them, including light itself.  This is because black holes have tremendous amounts of gravitational pull.  To make an analogy to help you better understand a black hole - imagine a golf hole at a golf course or miniture golf park and then imagine that hole having gravity far greater than the gravity that holds you to the Earth.  Then, imagine that as you got close to the golf hole you feel it pulling on you like a magnet pulls a paper clip that is nearby and the closer you get to the hole the stronger the pull.  If you were to get too close to the black hole, it would literally suck you into it - even though your body would be too big to fit.  It wouldn't matter because the gravitational pull would break you into pieces and pull you into that tiny little hole.  So that's how strong a black hole is.  So naturally, if they were to create a miniture black hole while colliding particles there would be some concern that it could cause immense damage to the facility and quite possibly start to grow larger and suck in bigger things.  

Proponents, on the other hand, state that if a miniture black hole were created (which they do not think could happen) it would be far too small to cause any damage and would dissipate quickly and disappear.  But I guess no one will know for sure until they fire that sucker up.  I, for one, don't think we have anything to worry about there because the Galactics would never allow something catastrophic to happen to us or Mother Earth.

Well, I hope I was able to help you to understand what the Hedron Collider is Suzen.

Namaste,
Quinn