Quantum field theory takes the basic methodology of electricity and magnetism (i.e. the use of fields as propagators of interactions) and formulates it in a way which produces the correct quantization of charge, angular momentum, mass, etc etc. It fully incorporates special relativity (which traditional 'quantum mechanics' doesn't).
QFT still does not include gravity. QFT and general relativity (which explains gravity) do not mesh-well; i.e. they contradict each other, and don't reproduce the other's results. Many people think that gravity will have to be quantized before these theories can be combined; this is the concept between Quantum Gravity---which does not exist yet.
M-Theory (which is an extension of, and very similar to, string theory) is a candidate theory of everything. I.e. it might include the three quantum forces, and gravity, with special relativity, and all of the bells and whistles are crazy other things. We don't know yet, because the mathematics is only vaguely understood by a handful of people in the world; and no one has really been able to pull everything together (if it can be done). The big problem with m/string-theory is that none of its current predictions are testable. The only way we (or at least that I'm familiar with) know to test string-theories, would be to use energies that may never be accessible (one of the classic examples: if you were to build a particle accelerator with high enough energy to test ST, it would have to be the size of the galaxy---or something like that).