Superstring theory is only 1 part of string theory. String theory tries to show that all particles comprise vibrations of strings. Superstring theory is the part of string theory that addresses

supersymmetry..

One of the early attempts at string theory required there to be 26 dimensions. However, that theory threw out spurious particles travelling faster than light. These were named tachyons. It turned out that there was a flaw in that theory and it was discarded. Despite the efforts of Star Trek writers, tachyons do not exist and any theory that contains them is filed under C for crap.

Various versions of string theory have been produced since. At the strings '95 conference,

Ed Witten showed that the 5 versions of string theory (three superstrings and two heterotic strings) were, in fact, 1 and the same and that it was equivalent (dual) to a theory of 11-dimensional supergravity. Witten called it

M-theory. As far as I am aware, 10D is the figure that string theorists currently work with.

However, later models concerned with warped higher dimensions have shown that there could be fewer or more.

For those interested in string theory,

here's an interesting site.

And here's a site with

lots of links.