String theory is an attempt to provide a unified description of all known physical forces. It replaces the point-like elementary particles of traditional particle physics by extended objects, the so-called strings. This replacement is expected to solve some serious technical problems that have marred all prior attempts to unify particle physics and gravitation. The seemingly innocent step from point particles to strings has complex and far-reaching mathematical implications, the investigation of which has driven the dynamic evolution of string theory since it was first proposed as a fundamental theory of matter in 1974. The emerging scenario posits a topologically complex 10- or 11-dimensional spacetime, traversed by multidimensional finite and infinite objects and tiny one-dimensional oscillating strings. At distance-scales accessible to present particle experiments neither the extra-dimensions nor the extendedness of strings are observable and string physics can be approximated in terms of point-like objects moving in three-dimensional space.