There are four fundamental forces in the universe: gravity, electromagnetism, and the weak and strong nuclear forces. Each of these is produced by fundamental particles that act as carriers of the force. One of the goals of Physics is to find a single theory that unites all of the four forces of nature. The first two are familiar. Electromagnetism is the force that holds a fridge magnet to a refrigerator while gravity is trying to pull it off towards the earth. The strong nuclear force is responsible for holding the central part of atoms (their nuclei) together, while the weak nuclear force is involved in the decay of these nuclei.
In the attempt to tie all the four forces together a lot of interesting ideas and new theories have been proposed. One of the most promising of these new theories is string theory. In attempting to unite gravity with the three other forces, string theory requires us to change the way we view the universe
The behaviour of all of these particles and forces is described with impeccable precision by the Standard Model, with one notable exception: gravity. For technical reasons, the gravitational force, the most familiar in our everyday lives, has proven very difficult to describe microscopically. This has been for many years one of the most important problems in theoretical physics-- to formulate a quantum theory of gravity.
General relativity and quantum mechanics take different approaches at looking at how the universe works. Many physicists feel that there must be a method that unites the two. One contender for such a universal theory is superstring theory, or string theory, for short. Let's take a brief overview of this complex perspective...
In the last few decades, string theory has emerged as the most promising candidate for a microscopic theory of gravity. And it is infinitely more ambitious than that: it attempts to provide a complete, unified, and consistent description of the fundamental structure of our universe. (For this reason, it is sometimes, quite arrogantly, called a 'Theory of Everything').
The essential idea behind string theory is this: all of the different 'fundamental ' particles of the Standard Model are really just different manifestations of one basic object: a string. How can that be? Well, we would ordinarily picture an electron, for instance, as a point with no internal structure. A point cannot do anything but move. But, if string theory is correct, then under an extremely powerful 'microscope' we would realise that the electron is not really a point, but a tiny loop of string. A string can do something aside from moving--- it can oscillate in different ways. If it oscillates a certain way, then from a distance, unable to tell it is really a string, we see an electron. But if it oscillates some other way, well, then we call it a photon, or a quark, or a ... you get the idea. So, if string theory is correct, the entire world is made of strings!
Perhaps the most remarkable thing about string theory is that such a simple idea works--- it is possible to derive (an extension of) the Standard Model (which has been verified experimentally with incredible precision) from a theory of strings. But it should also be said that, to date, there is no direct experimental evidence that string theory itself is the correct description of Nature. This is mostly due to the fact that string theory is still under development. We know bits and pieces of it, but we do not yet see the whole picture, and we are therefore unable to make definite predictions. In recent years many exciting developments have taken place, radically improving our understanding of what the theory is.
References: space.com; quantamagazine.org; physics.org; www.nuclecu.unam.mx; www.superstringtheory.com; www.nationalgeographic.com/