String theory

                    General relativity
General relativity theory tells us about how the gravity works in the macro-universe. It mainly tells us how the planets orbit stars and how these stars and planets orbit the Galaxy and so on.

                  Quantum Mechanics
Quantum Mechanics tells us about the micro-universe. It gives us description of atoms, electrons and other sub-atomic particles and how they all interact with each other.
       
           Scientists were trying to merge these two theories for a long time to make it is a unified theory due to some problems where these two theories don't agree with each other in principle. For example if we take black holes, which are extremely dense and are tiny forming singularity. So  which theory could be used to define among both the theories. Because relativity theory explains the extreme mass of black hole and quantum mechanics explains the singularity of black hole.
        String theory says that the basic constituent of all the matters is nothing but a one-dimensional vibrating string that is extremely small.  Hence string theory attempts to unify the fundamentals forces like electromagnetic force, strong and weak nuclear forces and gravitational forces.
        String theory explains these all by eliminating the idea of assuming subatomic particles as a point, instead as a tiny bits of energy vibrating which are hence called as strings. An average string is estimated to be about 10^–33cm long. According to this theory, these vibrating strings are as small as Planck's length  which is 1.616199×10^-35. At this scale, even electrons and protons are quite huge!. Since strings has finite size, they can vibrate.
               There is one important thing to note that for string theories to be consistent mathematically, they need to describe strings moving in more than four dimensions. Assuming the string theory to be correct, the extra dimensions are found to be hidden from our ability of detecting it.
                   Basically there are two kinds of strings:Strings can either be open or closed.
   • A closed string is a string that has no   end points and therefore is topologically   equivalent.
   • An open string on the other hand has two end-points and is topologically equivalent to a line interval.
          
           Note that not all string theories contain open strings, but every theory must contain closed strings, because interaction between open strings can always result in closed strings.
         
         Depending upon how these one-dimensional strings vibrate, and in what pattern and orientation they vibrate, they generate particles like quarks, electrons, photons and other elementary particles of mass. One of the vibration modes of a closed string can be identified as the graviton. In certain string theories the lowest energy vibration of an open-string is a tachyon and other vibrational modes of  open strings exhibit the properties of photons and gluons.
        
           But the original versions of string theory is the Bosonic string theory. It contains only bosons and hence it was so called. There some problems relating to Bosonic theory:
      *First it predicts only the existence of       bosons but there were many physical particles like fermions.
       * It predicts the existence of imaginary mass, implies that the theory is instable to a process known as tachyon condensation.
                  
               Hence later developed the superstring theory, which possess a connection between bosons and a class of particles called as fermions.
                  And M-theory is a theory in physics that unifies all consistent versions of superstring theory(type 1, type 2A, type 2B and two versions of heterotic string theory).....