M-theory (the "M" stands for the mother of all theories, magic, mystery, or matrix, depending on the source) is an adaptation of superstring theory developed by Ed Witten of Princeton and Paul Townsend of Cambridge. Townsend and Witten's version could potentially be the unified field theory sought by Einstein for the last 40 years of his life: a simple equation that would reconcile incompatible aspects of his theory of relativity and quantum theory to explain the nature and behavior of all matter and energy. Applications of this knowledge could, through unlocking nature's secrets, enable future technologies that currently are only spoken of within the realm of science fiction: an inexhaustible source of clean energy, and time travel, for example.
Superstring theory (sometimes just called string theory) has as its basic premise the belief that the four fundamental forces of nature (gravity, electromagnetism, and strong and weak nuclear forces), as well as all matter are simply different manifestations of a single essence. This essence, the material making up all energy and matter, is thought to consist of tiny (a hundred billion billion times smaller than the nucleus of an atom) vibrating strings that exist in a multi-dimensional (10 or 26 dimensions) hyperspace. The extra dimensions (beyond the ones we recognize: three spatial dimensions and time) are thought to be compactified, or curled up, into tiny pockets inside observable space. The particular vibrations of the strings within this multidimensional hyperspace are thought to correspond to particles that form the basis of everything - all matter and energy - in existence.