String theory is a newer theory in physics that attempts to explain everything. The currently used atomic theory has a flaw, you can keep asking "Well what are protons made of? Quarks? Well what are quarks made of?" etc etc.
String theory escapes this endless loop of going smaller and smaller. String theory states that quarks are made up of tiny vibrating strings. One may ask, "Well if all quarks are made up of tiny vibrating strings, what makes the quarks different?" (As the quarks must obviously be different because there have been dozens of different kinda of particles discovered (you know, alpha , beta , etc)). The answer to this question is that the string vibrate at various frequencies. This additional variable accounts for the many property differences of particles.
Thats the jist of string theory. Now for scientists to make this theory check out mathematically, they had to add 7 extra dimensions (for a grand total of ten) and there are lots of theories out there as to what these ten dimensions are. Some say they are too small for us to observe. If we ever do prove the existence of other dimensions (for example, if we observe matter disappearing (not converting to energy) then we can assume it has left our dimension, we will be much closer to solving the mystery of the universe. But then what about other universes...
Relativity collides with atomic theory, thus string theory is born!