The language of categories and functors is not only an indispensable tool in many areas of high-level mathematics, but is itself an incredibly interesting subject, worthy of study in its own right.

Based on notions developed in the 1940s by Mac Lane and Eilenberg, category theory (or "generalised abstract nonsense" as it is sometimes known) attempts to unify mathematics by describing similar structures through the subject in terms of "objects" and "arrows".