
One form of unified field theory involves explaining the basic units (quanta) of energy and matter in terms of tiny vibrating loops known as superstrings. But for the math of superstring theory to make sense it is necessary to postulate a hyperspace or higher spatial dimensions beyond the usual three. Hence the idea of ten dimensional space, eleven if you include time as well.
Superstrings would vibrate in an extremely small scale. Their hyperdimension motion would return to its starting point almost instantanously. A huge amount of energy would be required for a particle accelerator to detect such tiny dimensions (one of many paradoxes of quantum physics  the smaller you go to measure something, the more energy you need to generate).
In 1996, Dr. Joseph Lykken of Fermilab suggested that superstrings might have effects detectable at much lower energies. If so, the hidden superstring dimensions could be much bigger than originally thought, as much as a millimeter across. In these hidden dimensions, the 3 dimensions of our universe would be extremely thin (about 10millionth of a billionth of a millimeter) but these universes would exist in other dimensions, which are extremely thin relative to our universe.
Superstring Links and references 
The Official String Theory Web Site  by Patricia Schwarz  really easy to follow intro to Superstrings  accessible and enjoyable stuff  recommended!
SUPERSTRINGS! Home Page  an online introduction to superstring theory, which is the leading candidate for the theory of all fundamental interactions in the universe  by John M. Pierre
EventSymmetric String Theory  lots of stuff on superstrings and supersymmetry (unified field theory). Hardly any maths.  chapter 8 or EventSymmetric SpaceTime, by Philip Gibbs
Standard textbook on strings and superstrings. Although pretty well written (doesn't skip equations,
has very informative appendices and glossaries), this is for pros only. With it they should be able to follow
the literature (articles, preprints,..). An aside remark: since the discipline has not yet yielded any significant
experimental confirmation (its status still shaky); the math too exacting (and better presented in other books)
 it's virtually useless for outsiders.
rating:
accessibility  Very technical (advanced level textbook equivalent)
quality 