Multiple String Theories converge to a unifying M-THEORY !!

Strings

 

The physical idea is utterly simple. Instead of many types of elementary point-like particles, physicists postulate that in nature there is a single variety of string-like object. The string is not 'made up of anything', rather, it is basic and other things are made up of it. As with musical strings, this basic string can vibrate, and each vibrational mode can be viewed as a point-like elementary particle, just as the modes of a musical string are perceived as distinct notes!
String theory solves the deep problem of the incompatibility of the two fundamental theories (Einstein's General Relativity and Quantum Field Theory) by modifying the properties of general relativity when it is applied to scales on the order of the Planck length. Modern accelerators can only probe down to distance scales around 10^-16cm and hence these loops of string appear to be point objects.
However, the string theoretic hypothesis that they are actually tiny loops, changes drastically the way in which these objects interact on the shortest of distance scales. This modification is what allows gravity and quantum mechanics to form a harmonious union.
There is a price to be paid for this solution, however. It turns out that the equations of string theory are self-consistent only if the universe contains, in addition to time, nine spatial dimensions. As this is in gross conflict with the perception of three spatial dimensions, it might seem that string theory must be discarded.
This is, however, not true.

 

Multiple String Theories

There is however more than one string theory. These theories are classified according to whether or not the strings are required to be closed loops, and whether or not the particle spectrum includes fermions (particles that makes up matter). In order to include fermions in string theory, there must be a special kind of symmetry called supersymmetry, which means for every boson (particle that transmits a force) there is a corresponding fermion. So supersymmetry relates the particles that transmit forces to the particles that make up matter.
String theories that incorporate bosons only are no longer popular as they require 26 space-time dimensions and a particle with imaginary mass the tachyon. There are quite a few superstring theories that make sense mathematically which only require ten dimensions. A few of the differences between them include, theories with closed loops only and others with closed loops, which can break into open strings.
Theories with massless fermions only spinning one way (chiral) and string theories, which are heterotic, meaning right moving and left moving strings, differ. Different combinations of the above properties leave us
with 5 plausible (mathematically) theories.

 

M-Theory

There was a difficulty in studying these theories: physicists and mathematicians did not have tools to explore the theories over all possible values of the parameters in the theories. Each theory was like a large planet of which we only knew a small island somewhere on the planet. But over the last four years, techniques were developed to explore the theories more thoroughly, in other words, to travel around the seas in each of those planets and find new islands. And only then it was realised that those five string theories are actually islands on the same planet, not different ones! Thus there is an underlying theory of which all string theories are only different aspects. This was called M-theory.
One of the islands that was found on the M-theory planet corresponds to a theory that lives not in 10 but in 11 dimensions. This seems to be telling us that M-theory should be viewed as an 11 dimensional theory that looks 10 dimensional at some points in its space of parameters. Such a theory could have as a fundamental object a Membrane, as opposed to a string. Like a drinking straw seen at a distance, the membranes would look like strings when we curl the 11th dimension into a small circle.