oooh i like this question. String theory is this crazy theory that has been invented by the theorists that experimentalists can’t check with an experiment!
The basis of it is that everything is made up of tiny strings instead of point like particles. One thing that is good about this theory is that it manages to explain gravity (at the moment we cannot incorporate gravity into our standard model – that is the model we have of all the particles and forces). However it also requires extra dimensions which is a bit mad. It is hoped that someday string theory will make some predictions that we can actually test with an experiment.
Again this is a bit outside my comfort zone but as far as I understand it, string theory is an attempt to unite the two great theories of physics – general relativity (which explains much of the universe, including gravity, on a very large scale) and quantum mechanics (which is our best explanation of how things go on a small scale).
It has proved difficult to bring these things together and I think string theory is an imaginative idea to try to do that. I believe it is about thinking of space as quantised, that is of being made up of tiny tiny parts that are ultimately indivisible – and that particles are not point-like but made of little vibrating strings. One of the requirements of the theory is that there should be more that 4 dimensions in space-time. I think you have to have 10 or 11 for the theory to work.
However, no-one knows if the theory is correct. In fact I’m not sure if it has ever been tested which is not a good thing for a theory. What’s the point of a hypothesis if it doesn’t suggest experiments that you can do to check it? As a result, it has many critics.
We already know our universe has four dimensions up/down, left/right, forward/backward and the time at which you are standing in a given position, but some theories that describe the building blocks of everything around us suggest that the dimensions we can experience are not the whole story.
String theories suggest that there may be hidden dimensions in the universe that are too tiny for us to notice. The Large Hadron Collider at CERN ( http://www.lhc.ac.uk/ ) may well provide evidence for these extra dimensions by producing mini black holes which will exist for a tiny fraction of a second before vanishing in a puff of particles.
We don’t know if extra dimensions really do exist or are just a prediction of a theory that may well be proved incorrect, but the LHC may provide an answer to this question in the next few years.