Basic idea
The basic idea is that the fundamental constituents of reality are strings of the
Planck length (about 10−33 m) which vibrate at
resonant frequencies. Every string in theory has a unique resonance, or harmonic. Different harmonics determine different fundamental forces. The tension in a string is on the order of the
Planck force (1044
newtons). The
graviton (the proposed messenger particle of the gravitational force), for example, is predicted by the theory to be a string with wave amplitude zero. Another key insight provided by the theory is that no measurable differences can be detected between strings that wrap around dimensions smaller than themselves and those that move along larger dimensions (i.e., effects in a dimension of size R equal those whose size is 1/R). Singularities are avoided because the observed consequences of "
Big Crunches" never reach zero size. In fact, should the universe begin a "big crunch" sort of process, string theory dictates that the universe could never be smaller than the size of a string, at which point it would actually begin expanding.
Superstrings 
For the last 20 years of his life,
Albert Einstein was something of an oddity in the physics community, like a beloved eccentric uncle whose favorite subject of conversation draws embarrassed looks around the table. While quantum theory, the theory of the infinitesimally small, was being tested with accuracy never attained before, he refused to accept that it was the ultimate theory. For the last years of his life, he worked on a way to reconcile his own theory of gravitation and the quantum description of the world. He didn't succeed and died without seeing his dearest dream realized.
More than 40 years later, Einstein is almost vindicated: The long lasting problem of incompatibility between
general relativity and quantum mechanics seems to be on its way to a resolution. The solution may be difficult to grasp. If the handful of physicists involved in what are called "superstring theories" (or string, for short) are correct, we live in a world weirder than you can probably imagine.
It's a world of 10 dimensions, with some curled up at a microscopic level and some "big" dimensions that we perceive as "real." A world where the distinction between space and time is spurious (as taught by general relativity). A world where, in fact, the very notion of space and time is bound to disappear. In the words of Brian Greene, a professor at Columbia University and author of a book on the subject,"if string theory is correct, the fabric of our
universe has properties that would have dazzled even Einstein."
In string theory, there are no elementary particles (like
electrons or quarks), but pieces of vibrating strings. Each vibration mode corresponds to a different particle and determines its charge and its mass. In the current understanding of the theory, those strings are not "made of" anything: they are the fundamental constituent of matter. The consequences of replacing point-like particles by vibrating microscopic strings are enormous. The only consistent framework to describe those strings implies a 10- or even conceivably an 11-dimension world in which 6 or 7 dimensions are curled up. Those extra dimensions are the ones which determine the properties of the world we live in. The larger dimensions are what we perceive as the ordinary space and time.
A little reading for you Dipayankar,
Best,
Pat