Basically, around 1980 or so, theoretical physicists managed to unify the electromagnetic and weak forces into the electroweak force. This resulted in, among other things, the standard model. Feeling rather proud of themselves, several of them sat down and started working out a way to unify the new electroweak force with the strong nuclear force to create the electrostrong force. They gave this lofty goal the name "Grand Unified Theory", or GUT. Very quickly, they discovered that most of their workable GUTs seemed to make some rather startling predictions, namely leptoquarks (a new class of boson with both a quark-like fractional electric charge and QCD color), proton decay, and magnetic monopoles. Unfortunately for the physicists, none of these have been observed to date.
The current hot topics in theoretical physics are string theory and supersymmetry, which try to unite all four fundamental forces together. Hopefully, this will bypass the controversy surrounding GUT.
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