Unified Field Theory, also referred to as the Grand Unified Theory in its more complex state, is a physical theory that attempts to unite the four known forces in nature - the Electrostatic, the Strong Nuclear, the Weak Nuclear, and the Gravitational. Unifying these four forces under one single set of mathematical expressions was long considered the 'Holy Grail' of modern physics, and absorbed the efforts of thousands of physicists for the better part of a century.
Unified Field Theory started out as Albert Einstein's attempts to reconcile his own view of the universe with the emerging field of Quantum Mechanics. Einstein despised quantum theory in its infancy, perceiving it as a threat to the foundations upon which his theories of Special and General Relativity were built. His famous statement, "God does not play dice with the universe", is a reflection of his disposition towards a probabilistic universe. By contrast, all of Relativity is based on Newtonian determinism. Einstein was partially successful in unifying these wildly different forces, but ultimately failed. Heisenberg later took up the gauntlet for the more noble reason of preserving the elegance of physics - he believed that the ultimate goal of physics would be to great an 'equation for everything'. Currently however, there is no theory of gravitation at the quantum mechanical level.
Interest in the Unified Field Theory waned as some of the greatest minds of the twentieth century fell in pursuit of the ultimate equation. Research was recently rekindled however with the theorization of the Higgs Boson, the currently theoretical particle thought to give particles mass. The reason for this is difficult to explain in words, but I will try. Under the Standard Model of particle physics and our current understanding of field theory, we can plot graphs describing each of the four forces. If we plot them on the same axes, at increasing energies, the four lines begin to converge to a point, but without including the Higgs Boson, they miss convergence by a tiny but significant margin. By including the Higgs into the Standard Model, it is possible to unify all four fields, at a very high energy state known as the Unification Energy. This energy level also gives rise to several other 'fringe theories' such an over-unity (free energy) and Magnetic Monopoles.
Currently, the conclusion of the UFT, for good or ill, is still a dream for physicists. However, the construction of ever more powerful particle accelerators may eventually change this situation, perhaps leading to the final discovery of the Higgs and beyond.