Although there are countless other experiments either in-progress or planned (such as the recent faster-than-light Neutrino experiment), the primary function of the LHC is to find the Higgs Boson. As part of the LHC, two very different experimental stations, called ATLAS and CMS are the main foci of the research.
The Higgs Boson was theorised by Scottish Physicist Peter Higgs (amongst others) in 1964 as a means of attributing mass to all atomic particles. 'Boson' is a general term for a range of sub-atomic particles which mediate the fundamental forces of the universe; the best known of which is the photon, which mediates electromagnetic forces.
The problem with the Standard Model of particle physics was that, prior to Peter Higgs' work; there was no mechanism whereby any form of mass could be ascribed. We had an understanding of the weak and strong nuclear forces and also of gravity and electromagnetism, but not mass.
Higgs and his fellow researchers postulated a particle which would be of such high energy that it could not be detected by current systems (with the exception of the US-based Tevatron which had its capabilities stretched to indulge in a decade-long parallel search for the Higgs). This was the primary driver to build the LHC.
In April 2011 and again in December that year, the world of Physics was tantalised by some preliminary results coming out of both the ATLAS and CMS experiments to suggest the Higgs had been discovered.
Higgs originally postulated the eponymous particle to have a mass of between 115 and 150 GeV (Physicists measure the mass of particles in terms of their energy level - thus GeV refers to Giga-electron volts; an electron volt is related to a simple unit of energy in an electron). The ATLAS experiment suggested 126GeV and CMS 124GeV. Neither had sufficient data to make an assertion at the confidence required by Physicists.
It is generally regarded that the confidence level for the data must be at least 5 standard deviations (one chance in 3.5M that the result was a random fluke) before being accepted as 'true'. The December data from ATLAS was at 2.3 standard deviations and CMS' data was somewhat lower.
Many news outlets with access to those "in the know" from the ATLAS and CMS teams are reporting a high level of excitement, with statistical confidence levels being reported between 4.5 and 5 standard deviations; just short of the required 5.
We await the 5pm announcement with great interest.