M-theory is a theory in theoretical physics that unifies all consistent versions of super string theory


a two-dimensional hypersurface of the quintic Calabi-Yau three-fold. This is a way of representing 2D strings wrapped up on itself. image: wikipedia

A unified string theory was first conjectured by Edward Witten at a string theory conference at the University of Southern California in the spring of 1995. Witten's announcement initiated a flurry of research activity known as the second superstring revolution.

Prior to Witten's announcement, string theorists had identified five versions of superstring theory. Although these theories appeared at first to be very different, work by several physicists showed that the theories were related in intricate and nontrivial ways. 

Physicists found that apparently distinct theories could be unified by mathematical transformations called S-duality and T-duality. Witten's conjecture was based in part on the existence of these dualities and in part on the relationship of the string theories to a field theory called eleven-dimensional supergravity.

A schematic illustration of the relationship between M-theory, the five superstring theories, and eleven-dimensional supergravity. The shaded region represents a family of different physical scenarios that are possible in M-theory. In certain limiting cases corresponding to the cusps, it is natural to describe the physics using one of the six theories labeled there. image: wikipedia

Adapted from https://en.wikipedia.org/wiki/M-theory