Grand Unified Theory
Grand Unified Theories, or GUTs (without which, is has been alleged, there is no glory) are theoretical attempts to unify the electromagnetic, strong and weak forces. Despite their name, they are actually less "grand" than Unified Field Theory (which hopes to add gravity into the mix as well).
[edit] The Basic Idea
The value of the coupling constants in Quantum Electrodynamics (which describes the electromagnetic force), Quantum Chromodynamics (which describes strong force) and Quantum Flavordynamics (which describes the weak force) all vary as a function of the energy at which a particular interaction takes place. Assuming only the existence of the particles in the standard model, these values almost converge to a single number, but not quite. When every particle is assumed to have a supersymmetric partner, however, these values converge exactly.
[edit] Experimental Evidence
With the detection of the Intermediate Vector Bosons W+, W-, and Z0, the electromagnetic and weak forces have already been unified. This unified force is called the electroweak force. What is still missing, and thus preventing this unification from being a grand unification, is a unification of the strong force with the electroweak force.
As noted above, GUT depends upon every particle having a supersymmetric partner (i.e, all fermions have a corresponding boson and vice-versa). So far, there has been no evidence of this. Direct observation, on the other hand, is right out of the question, as the energy-levels needed for unification are well beyond what is currently technologically possible (or anything on the drawing board for that matter).