Sting theory = complex mathematical construction pretty much explained physically the same everywhere by eveyone -- n dimensions, n-4 wound up in the 4 "macroscopic" dimensions we live in. Lots of complex math that can be taken in many ways. There are a bunch of string theories with different maths and number of dimensions. Collectively known as M-theory.
Multiverse = not sure if it's a technical theory per se. Probably just goes off of mathematics of basic quantum mechanics; to wit, the probabilistic universe. Everything's probabilities. Not sure where they get the everything that can happen does so in infinite quantum mechanical universes... I haven't seen where they pull that from. Might be a general thing coming from the theory.