Studying categories and functors is not just studying a class of mathematical structures and the morphisms between them but rather the "relationships between various classes of mathematical structures". This fundamental idea first surfaced in algebraic topology. Difficult "topological" questions can be translated into "algebraic" questions which are often easier to solve. Basic constructions, such as the fundamental group or the fundamental groupoid of a topological space, can be expressed as functors to the category of groupoids in this way, and the concept is pervasive in algebra and its applications.