string theory is a theory concerned with particles and how they interract, I think it is just that right now...a' theory' a mathematical hypothesis. I think that when they start that LHC machine in europe it will maybe give some answers to string theory. That is due to start next week!!!!
If you look at a periodic table you will see that there is a pattern laid out. Different elements have similar properties.
Although at first we knew of nothing smaller than an atom it seemed that there had to be an underlying structure giving rise to these patterns.
Eventually of course we discovered protons and neutrons and electrons and learnt what made up atoms and how these patterns arose.
In the same way there are patterns in the fundamental particles we have today. 3 generations of quarks and "leptons". Quarks having properties that we call colour (although that's an analogy they're not really coloured)
So we feel there must be an underlying structure giving rise too this.
String theory is an attempt to describe this based on almost unimaginably small particles "strings" that have length but no width. The manner of their vibration giving rise to the particle types.
It's quite sucessful in explaining things but as far as I know has never been able to make a prediction that could be tested experimentally.