The Standard Model of particle physics describes twelve building blocks of the universe (six types of quarks and six types of leptons) and the four forces that control them (gravity, electromagnetism, and the weak and strong nuclear forces). The model beautifully describes their behaviour, with one exception: gravity. Recently, string theory has been postulated as the answer to a microscopic theory of gravity, but it goes even further and ambitiously attempts to become to the Theory of Everything: the unified description of the fundamental structure of our universe. String theory is based on the concept that the elementary particles of the Standard Model are composed of subatomic ‘strings’, and the strings’ vibrations determine the way particles interact. Imagine a guitar string—the note it makes depends on how it is plucked and how much tension it holds. Particles are the ‘musical notes’ of string theory, and how they interact depends on the oscillation of the strings. However, the strings are of the Planck length, which is 10^-33 cm—far too small to detect using most particle physics technology, so there is no experimental evidence showing that string theory is correct yet. Oh, but if it is correct, it means that several extra dimensions exist—in M-theory, which attempts to unify the five different string theory models, there’s 11 whole dimensions.
Check out Discover Magazine’s interview with physicist Brian Greene