Try to come up with a revised purpose section for LHC. This has been partly motivated by the need to stop the edit war that had been raging for a month or two, and also of course to get a better section, with something useful for the lay person who knows nothing, and also some meat for the pedestrian scientist who knows a little and wants to deepen his understanding. It cannot be comprehensive of course, but should link out to the major subtopics and try to put them into context.