String theory arose about thirty or so years ago but has come into the 'fore' during the last decade or so as physicists have attempted to search for a unified theory of the four forces of nature. Please allow me to explain why string theory has provided such a good starting point for a possible unification of the four forces!
Grand Unification Theory or GUT for short is an attempt by theoretical physicists to provide a single unified theory of the four forces of nature.
These four forces are: the strong nuclear force, a short ranged powerful force responsible for the binding together of nuclei; the weak nuclear force, a short ranged less powerful force responsible for radioactive decay processes; the electromagnetic force, a infinite ranged force of moderate strength responsible for chemical and electromagnetic wave phenomena; and last of all, the gravitational force, which governs the universe as a infinite ranged force that is only powerful when it results from massive bodies such as stars.
During the nineteen-sixties, the theoretical physicists Glashow, Salam and Weinberg developed a theory which unified the electromagnetic and the weak nuclear forces. This theory is known as the ‘electroweak’ theory, it predicted the neutral vector boson Z0, and weak nuclear force reactions arising from its exchange, in what are known as neutral current reactions. The theory also accounted for the heavy charged bosons W+ and W-, required for the mediation of all observed weak interactions, known as charged current reactions. These particles were discovered in 1983.This unified theory is a ‘gauge invariance’ theory, which means that if the components of its underlying equations are transformed, in position or potential, they still predict exactly the same physics. Because the force carrying particles (Z0, W+ and W-), of this theory, are massive spin-1 bosons a spin-0 boson is required to complete the theory. This spin-0 boson is the as yet unobserved ‘Higgs’ boson.
The discovery of the Higgs at CERN's LHC will help complete the standard model of elementary particle physics. This model deals with the quarks and their strong nuclear force interactions, along with weak nuclear force interactions and electromagnetic interactions among the leptons. However, the standard model doses not sit comfortably with Einstein's General Theory of Relativity (1915). This theory describes gravity as a geometrical distortion of space-time due to the presence of mass energy within it. Thus, even when the Higg's boson has been discovered, as surely it will, there will still exist a disparity between the force of gravity and particle theory.
The strong nuclear and gravitational forces have yet to be unified with the weak and electromagnetic. This is where string theory comes in.
A string is one of the main objects of study in string theory, a branch of theoretical physics. There are different string theories, many of which are unified by M-theory. A string is an object with a one-dimensional spatial extent, unlike an elementary particle which is zero-dimensional, or point-like.
By postulating this one-dimensional structure, many desirable features of a more fundamental theory of physics automatically emerge. Most notably, almost any theory of strings consistent with quantum mechanics must also contain quantum gravity, which had not been described consistently prior to string theory.
The characteristic length scale of strings is thought to be on the order of the Planck length, the scale at which the effects of quantum gravity are believed to become significant:
On much larger length scales, such as the scales visible in physics laboratories, such objects would be indistinguishable from zero-dimensional point particles. However, the vibrational modes and structure of the tiny string would be manifested as different elementary particles in the standard model of quantum field theory. For example, one state of the string would be associated with a photon, and another state with a quark. This unifying feature of string theory is among its greatest strengths, however no known solution of string theory exactly reproduces the particle content of the standard model.
This, then, is where the problem lies with the development of a GUT based upon string theory(s). There are at present five competing versions of super-string theory - none of which truly represent the physical world we see and measure.
I hope this helps, although I concede that it is a very brief survey, which requires a textbook level of coverage to do it justice!