ABSTRACT
Categories are one of the most important unifying concepts of mathematics. In particular they allow to create bridges between various parts of mathematics. Categories were introduced by Eilenberg and MacLane partly motivated by work in homological algebra. A further input in the development of the concept was given by work of Grothendieck in algebraic geometry. Here we will only explore the definition of categories and we give some examples. We begin with the simpler concept of correspondence.
