What is a “String Theory”?

Every particle is made up of molecules and atoms. If one probes deeper, an atom consists of electrons, protons and neutrons. Modern physics have revealed that these too are not the basic building blocks. These particles are also made up of quarks. Finally string theory talks about the most basic particles, tiny bits of vibrating loops or strands called the Strings. So small are these strings that if we visualize an atom to the size of a solar system then the strings would only be to the size of a tree.
Matter and energy are by the virtue of these tiny vibrating strings. Just like a guitar produce different tunes when plucked differently, different patterns of vibration of these strings create all sorts of matter and energy. In this view, our universe is actually the manifestation of a Grand Cosmic Symphony.
The very existence of string theory is to bridge the gap between quantum mechanics and general theory of relativity. GR is applicable to gigantic stars and galaxies while QM is applicable to microscopic particles. GR is not applicable when applied to microscopic particles; likewise QM is not applicable to gigantic stars and galaxies. The greatest challenge to physics is to come with answer as to which theory is correct because both these theories describes about the same universe and it can’t be true that both of them are correct. Both can’t be true because GR tells that the fabric of our universe is smooth and majestic while QM describes the fabric to be frantic, jittery and violent.
Before string theory came into light, the smallest particle of matter was viewed as a point particle dancing frantically in a microscopic world because of which the microscopic world is a violent place as described by QM. If strings are brought into consideration, the fabric turns out to be an intermediate between that described by QM and GR i.e. it is not so smooth or not so frantic. Thus by remodeling the fabric of the universe one can bridge the gap between QM and GR, the greatest theories of 20th century.

P.S: This is an article by a layperson. If one wants to have a good picture on the subject, I suggest one to read “The Elegant Universe” by Brian Greene.