String theory is an expansion on the module Quanta to Quarks. It's not actually in the Physics syllabus.
String theory is an alternative to the current "Standard Model" of particle physics. The Standard Model says that zero-dimensional points - tiny particles called quarks, leptons, and gluons - are the building blocks of all matter. String theory basically states that the fundamental constituents of matter are one dimensional extended objects, i.e. strings. These strings are believed to be of Planck length (about 10-35m) and oscillate at resonant frequencies in 11 or so dimensions. Yes, that's 11 dimensions, 7 more than we can ever comprehend.
The theory arose because of various problems with the current Standard Model. The Standard Model says nothing about gravity, for example - nothing in the Model explains why apples fall off trees. The Standard Model also can't explain mass - in the Model a particle called the "Higg's Boson" has been inserted to account for mass. We haven't actually found this particle yet, but particle physicists are searching away for it. So basically, the Standard Model is as good as we've got, but it's pretty ugly and has a lot of holes. As with anything that's ugly and holey, people have been searching for better solutions, and string theory is one of those. In fact, string theory is regarded as a possible theory of everything; a theory that can explain all of the mysterious forces at work in our world - gravity, electromagnetism, quantum mechanics, everything. The number of dimensions that the strings vibrate in (11, remember?) allows particle physicists to pull together quantum laws (the world of the very small), gravitational laws (the world of anything larger than very small), and relativity (the very very fast) into one comparatively neat package. Well, much neater than the Standard Model is at the moment, anyway.
However, string theory is still in its very early infancy. It's extraordinarily hard to see anything that's 10-35m long, let alone some strings that size. The theory requires much, much more development before it can be accepted or falsified - like a lot of particle physics, it's very, very theorectical at this point. Who knows what it'll turn out to be in the future....
Hope that helped,
I_F