Question: what is the 'string theory'?

Philip Ratcliffe answered on 14 Nov 2014:

Now this is a big big question; so I really don’t know if can answer it well enough to be understood, but here goes.
The idea in the start was that particle like protons, which are made of quarks, behave as though the quarks where tied together by an elastic string. We know it’s not really the way they are held together but it’s like that. The point is we don’t really know how they are held together, but we do know the physics of elastic bands. And this turns out to give quite a good description of protons, neutrons and many other particles we know of.
Then in the seventies two theorists who worked on these ideas (Mike Green in London and John Schwartz in the US) realised that a particular version of such theories (so-called superstrings) could also somehow describe more particles and also the graviton (which is presumed to be responsible for gravitational attraction, but has never been seen). They were joined by a brilliant mathematical physicist from the US called Ed Witten (he later won the Fields Medal, the maths equivalent of the Nobel Prize) and the theory started to be studied by many physicists.
Unfortunately, we still seem to be a very long way from being able to solve the theory and so we don’t yet know whether r not it really works. That is, it might be a very beautiful mathematical toy, but maybe it doesn’t actually describe the physical world in which we live.