String theory is an ambitious project that attempts to describe all known forces and particles in a single quantum theory. The initial premise is to replace the point particles of quantum field theory by small strings - objects with a length. By applying the standard laws of quantum mechanics, one finds that a particular vibrational mode of the string always describes the gravitational force, and moreover at low energies the theory reduces to a version of Einstein's general theory of relativity. One also finds that the strings can propagate only in ten spacetime dimensions - one time and nine spatial dimensions!
One of the reasons why string theory has come to dominate theoretical physics in recent years is that it has turned out to be related to so many other areas of fundamental physics, often giving new insights into those areas, and generating new relations between them. For example, the types of gauge theories that so accurately describe particle physics automatically appear in string theory, and do so in a number of distinct ways; many new results in quantum field theory, and relations between field theories, have been understood via their embedding in string theory. Perhaps even more remarkably, string theory also uses a vast amount of contemporary mathematics, particularly geometry, and this has had a huge impact on some areas of pure mathematics. It is this all-encompassing nature of the subject that makes it so compelling.