Types of string theory
Originally it was thought that there were five distinct string theories in flat 10D space, each of which could provide a starting point for compactifications to four dimensions. SO(32) Type I theories are distinguished by having open as well as closed strings, where SO(32) represents the symmetry group of the theory. Type IIA and IIB theories have only closed strings. Closed strings have no ends and are like closed rubber bands with the topology of a circle, while open strings have two free ends that can move. Waves on open stings bounce back and forth between the ends, but waves on closed strings circulate endlessly around the string in one of two possible directions (hence the two versions of Type II string theory). Finally there are two "heterotic" theories - SO(32) heterotic and E8 x E8 heterotic - which allow different kinds of waves to move in the two possible directions. It is now thought that the five different types of string theory are related to each other by deep symmetries such as T-duality, as if they were each the "classical" limit of a more fundamental theory called M-theory.