String theory is the idea that the microscopic constituents of matter are incredibly tiny bits of string. The string is like a guitar wire that undergoes only certain vibrations. For a guitar wire, these correspond to different musical notes. For the string, each vibrational mode is an elementary particle: one vibration is an electron, another is a quark, another is the photon (the particle of light) and so on. The fundamental forces are created by exchanging bits of string. The electric force, for example, is produced when a string in its photon mode is repeatedly "passed back and forth" between two electrically charged strings. Strings are able to accomplish this exchange because they can join and split: two strings are able to fuse ends to form one string (similar to tying together two shoe laces), or one string can break into two (similar to cutting a long shoe lace into two shorter pieces).
Some theorists are excited about string theory because its offers the ultimate unification. All forms of matter and all types of forces are manifestations of a single structure. String theory also has the potential to solve one of the greatest problems of theoretical physics of the 20th century: the incompatibility of quantum mechanics and Einstein's general theory of gravity.
A brief history of string theory is provided in Chapter V of The Bible According to Einstein's The twelfth book of Chronicles, called Moments in Modern Science.