String theory: what is it?

This is an attempt to describe the subject and the methodology of string theory as we understand them today, i.e., the entire set of problems which attract attention of theorists working in the field. The string model of Grand Unification of fundamental interactions is briefly discussed along with a broader string scenario of the unified field theory, a more mathematical concept, designed to facilitate understanding of the generic features of equivalence classes in different models of quantum field theory. A concise glossary of the most important notions unusual in physical literature but frequently used in papers on string theory is also included.