You might find the responses in this very old thread helpful. Someone asked basically the same question you did. I think the conclusion we came to was something like:
- "Strin[g] field theory is a second quantized formulation of string theory... this is a very technical point to explain. In a nutshell, string field theory is a more general form[al]ism to do calculations in string theory. It's more of a technique than a new theory."
- M-Theory is a variant of string theory with new structures, like an 11th spacetime dimension and the introduction of n-dimensional "branes" in addition to vanilla strings. Pre-M-Theory string theories can be found "inside of" M-Theory (that is, they are dual to special cases of M Theory).
- The two do not have anything to do with each other.
As far as I know String Field Theory techniques not widely used, but M-Theory has been the basis of virtually all string theory research for at least the last decade.