A classical string is a onedimensional extended object existing in fourdimensional spacetime. A typical example of a classical string is a piece of thread.
A classical string is specified by the string tension T, and can exist in a spacetime of arbitrary dimension. Quantum strings can only exist in certain critical spacetime dimensions such as 26 and 10. Quantum bosonic strings can be made supersymmetric yielding the superstring.
All of physical reality is made out of different states of the superstring. Roughly speaking, each vibrational mode of the string can be thought of as a point particle. Hence, one superstring gives rise to infinitely many local fermion and boson fields. All of the observed bosons and fermions can be cosidered as a vibrational mode of the fundamental superstring. It must be noted that the string is both constituent and interaction. Superstrings can be either open or closed. 

Quantum strings inherently contain a quantum theory of gravity! For this purpose, the size of the strings must be of Planck length scale.
Consider a particle at the point x_{0}. Conventionally, the properties of the pointparticle are then specified by a field A(x_{0}).
However, for a string, we have to determine infinitely many points (along the whole string). This is fundamentally the reason why a single string in effect yields infinitely may pointlike particles.
According to the big bang theory, we have the following scenario : Universe starts from a point > Strings > Inflation > Gravity differentiates > Further differentiation. Since strings occur before any differentiation, they must of Planck scale (10^{35}m), the distance at which all the forces are equally strong. 

Note : Distances are inversely related to energy by the uncertainty principle (the smaller the distance, the higher the energy scale). 
Last updated: 03 March, 2000