The theory of graphs.
A graph is a set of vertices connected by edges. The edges may or may not be directed.
One of the famous examples used in GraphTheory is the BridgesOfKönigsberg.
GraphTheory is widely used in computer science for things like call graphs, Algorithms and so forth. GraphTheory relies heavily on SetTheory, and many common algorithms involve partitioning (or colouring) vertices into sets.
One page links to GraphTheory:
