M theory is a name for a more unified theory that has the
different string theories, as we know them, as limits, and which
also can reduce, under appropriate conditions, to eleven-
dimensional supergravity. There's this picture that we all have
to draw where different string theories are limits of this M
theory, where M stands for Magic, Mystery or Matrix, but it also
sometimes is seen as standing for Murky, because the truth about
M theory is Murky. And the different limits, where the main
parameter simplifies, give the different string theories -- Type
IIA, Type IIB, Type I, and there's eleven-dimensional
supergravity, which turns out to be an important limit even
though it isn't part of the systematic perturbation expansion,
then there's the E8XE8 heterotic string, and there's SO(32)
heterotic string. So M-theory is a name for this picture, this
more general picture that will generate the different limits
through the different string theories. The parameters in this
picture we can think of being roughly hba