M-theory is a theory conjectured to exist by Edward Witten of the Institute for Advanced Study, that would unify the five different superstring theories: Type IIA string theory, Type IIB string theory, Heterotic string theory, SO(32) string theory, and heterotic E8×E8 string theory; all of these seemingly different string theories are simply different limits of the same underlying M-theory. Edward Witten has used M-theory to explain a number of observed dualities, such as the T-duality, U-duality and S-duality. The discovery of M-theory is hailed by string theorists as the second superstring revolution.
In M-theory, the fundamental constituents of the universe are membranes, or p-brane where p is the dimensionality of the membrane. Thus the strings theorized in String theory belongs in the class of 1 dimensional membranes, the 1-branes.
So far, no one has found any relation between M-theory and gravity, electricity, photons, or any other known aspect of the real world, nor has anyone shown that anything can be consistently calculated in M-theory.