String Theory is a relatively new field of theoretical physics. Put very simplistically, String Theory states that all matter in the universe, regardless of its outward nature, is composed of tiny vibrating strings of energy, referred to (appropriately enough) as cosmic strings. As cosmic strings vibrate, they cause quantum fluctuations in the space around them, and depending on just how these vibrations and fluctuations play out, a different particle may be formed (e.g. one type of vibration will cause the creation of electrons, another the formation of neutrinos, still others the formation of different types of bosons, etc., etc.). The exact nature of these strings is difficult to study, however, as they are incredibly tiny. (If a single proton were scaled up to the size of the entire universe, the cosmic strings inside it would only be roughly the size of trees; consequently, studying them directly would require incredible quantities of energy and a particle accelerator with a diameter roughly equal to that of the Milky Way galaxy, something which would obviously be nearly, if not totally, impossible to construct and would certainly be beyond any technology we currently have.) While String Theory remains highly speculative and controversial at this point, it has the potential to unify the theories of General Relativity and quantum mechanics, explain just how gravity works at the quantum level, explain what occurs within black holes, explain how and why the Big Bang occurred, and much, much more. It is therefore perhaps unsurprising that many religious officials regard the entire concept with skepticism and insist that the is laughable.
String Theory is also related to several other new fields, such as M-Theory.