It takes a while to get a grip on these string, Mercedes. Search Google and you find stuff like this:
http://library.thinkquest.org/27930/stringtheory1.htm
"For reasons that will later become apparent, the universe seems to incorporate closed strings, although open strings may also be present. Recent research has also revealed that "strings" may actually have many different dimensions, from the one-dimensional strings originally postulated to a two-dimensional membrane to many analogous structures in higher dimensions. Physicists have taken to calling strings "branes," and defining each's dimensional extent with a number, i.e a one-brane is a one-dimensional string..."
