Basics of String Theory

                   A theory of everything (ToE) or final theory or string theory is a putative theory of theoretical physics that fully explains and links together all known physical phenomena, and predicts the outcome of any experiment that could be carried out in principle.The term mainly refers to the desire to reconcile the two main successful physical frameworks, general relativity which describes gravity and the large-scale structure of spacetime and qantum field theory particularly as implemented in the standard model which describes the small-scale structure of matter while incorporating the other three non-gravitational forces, the weak, strong and electromagnetic interactions.

A primary stumbling block in the construction of theory of everything  is that straight forward attempts to apply quantum mechanics to the gravitational field, in the same way as for say the electromagnetic field, fails due to the breakdown of the renormalization procedure. Thus the central issue is how to combine general relativity and quantum mechanics. This is  also one of the unsolved problems in physics.