I'm not just saying there are other more than four dimensions because it makes me happy 

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String theory is a term used to describe a set of very closely related mathematical models of elementary particles and their interactions. String theories seek to unify the theory of gravity (general relativity) with the three other forces of nature which we have learned to describe using the techniques of quantum field theory.

In string theory the known elementary particles are no longer described as dimensionless mathematical point-objects but rather as extended one-dimensional objects (hence the name 'string'). These objects may be either open bits of a line or closed into loops. The size of the individual strings is so fantastically small that any experiment we could possibly perform on an elementary particle would not reveal its string-like nature -- it would look just like the point-particle we expect.

Since the strings have a finite size they can vibrate. All the known particles of nature are just different modes of vibration of the string. Thus the string is the only truly 'fundamental' particle.

For string theories to be mathematically consistent, they need to describe strings moving in more than four dimensions. If a string theory is the correct theory of nature, these extra dimensions must obviously be hidden from our ability to detect them. The general assumption is that they are 'compact' -- rolled into dimension so small that our every-day experience only reveals the four large ones (three space, one time) in which we live.

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Leaping to higher dimensions can also simplify the laws of nature. In 1915, Einstein changed completely our notion of gravity by leaping to the extra dimension of time. In 1919, the German mathematician Theodor Kaluza added a fifth dimension and in so doing unified space-time with Maxwell's equations for electromagnetism. This triumph was largely forgotten amid the frenzy of interest generated by quantum mechanics. Only in the 1980s did physicists return to this idea to create superstring theory.

In superstring theory, the subatomic particles we see in nature are nothing more than different resonances of the vibrating superstrings, in the same way that different musical notes emanate from the different modes of vibration of a violin string. (These strings are very small-of the order of 1035 metres.)
Likewise, the laws of physics -the forces between charged particles, for example-are the harmonies of the strings; the Universe is a symphony of vibrating strings. And when strings move in 10-dimensional space-time, they warp the space-time surrounding them in precisely the way predicted by general relativity. So strings simply and elegantly unify the quantum theory of particles and general relativity. Better still, gravity is not an inconvenient add-on. "Unlike conventional quantum field theory, string theory requires gravity," Witten has said. "I regard this fact as one of the greatest insights in science ever made."

But, of course, all this takes place in 10 dimensions. Physicists retrieve our more familiar 4-dimensional Universe by assuming that, during the big bang, 6 of the 10 dimensions curled up (or "compactified") into a tiny ball, while the remaining four expanded explosively, giving us the Universe we see. What has consumed physicists for the past ten years is the task of cataloguing the different ways in which these six dimensions can compactify. Their task has been especially difficult because mathematicians have not worked out the topology and properties of these higher-dimensional universes. The physicists have had to blaze the trail and invent entirely new areas of mathematics. These efforts have revealed millions of compactifications, each of which yields a different pattern of quarks, electrons and so on.

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