For category theory, from Abstract and Concrete Categories: The joy of cats, by Adámek, Herrlich and Strecker, page 12:
(http://katmat.math.uni-bremen.de/acc/acc.pdf)
Categorists have developed a symbolism that allows one quickly to visualize quite complicated facts by means of diagrams.
For me, this represents the fact that most, if not all of mathematics, is about structures and relations: even the simplest of them, when combined and interrelated, can give birth to fairly complex behaviour.
