QCD (Quantum Cromodynamics) is believed to be the theory for strong interactions. However, despite its successes to describe high energy processes it has not been possible to do calculations at low energies (e.g. calculate the mass of the proton). It is believed that a large N (N is the number of colors) approximation would simplify the theory. Furthermore, this simplified theory would be a free string theory.
We consider a supersymmetric version of QCD where one can find this string theory quite explicitly. The strings move in a higher dimensional space, and they describe a quantum theory of gravity in this higher dimensional space. This leads, on the one hand, to an explicit solution for the large N dynamics of this theory, and on the other hand, to a description of quantum gravity, quantum black holes, etc., in terms of a conventional field theory.