As a quick overview, string theory is a branch of theoretical physics which seeks to unify Einstein’s general theory of relativity with quantum mechanics–the two most successful theories developed in the 20th century (more accurately, the standard model and GR) by supposing that fundamental particles are actually Planck-length 1D vibrating somethings (string is really an analogy) in 8 or so extra dimensions that are about the same size as the strings. This setup allows the strings to do all sorts of bizarre things and by changing their vibrations they can change particles.
But there are a few problems with the theory:
1) The string theory equations are approximations–they cannot be solved accurately
2) The strings and dimensions are unobservable
3) There are a huge number of possible string theories, only one of which corresponds to our real universe.
4) String theorists so dominate university systems that competing theories hardly have a chance to break in to the popular consciousness, as a result young physicists and the general public think that string theory is the sole idea that unifies physics.
Of the first problem, not much can be done. Approximations are not problematic in physics, save when trying to make definite predictions. My knowledge of mathematics is limited enough that I’m unsure what string theory is doing with its approximations. To be precise in regards to the second problem I would like to point out that the strings are in fact theoretically predictable, it’s just that the energy required to probe the Planck length is so huge that it will likely remain outside our reach for a long, long time. But because it is so large, the predictions of string theory remain unobservable and therefore unscientific.
The third problem is the most sticky and is used to justify the multiverse theory: if string theory is correct then each possible universe corresponds to an actual universe and so our universe is not unique or special, it just “wins the cosmic jackpot” as Paul Davies puts it. However, I would argue that the ability to change some key variables doesn’t imply that every permutation is a different universe. Authors come up with new universes all the time, but nobody would seriously suggest that they were real universes except perhaps Nick Bostrom ( http://en.wikipedia.org/wiki/Simulation_hypothesis). Furthermore all proposed multiverse theories maintain that the other universes cannot interact with each other, making them unfalsifiable and thus unscientific. Another argument is that there is no mechanism that determines the laws of physics in a universe, so the multiverse theory makes no sense if each universe must be unique. There is, however, a hypothesis out there that solves those problems: God creates one universe. By create I mean selects the laws of physics, actually makes matter, etc. Paul Davies writes that the “God Hypothesis” is unscientific because it doesn’t answer the question of how. That is completely untrue, because all of the natural sciences combine to answer the question “How did God create the universe and how does He rule it?”