In graph theory, a graph is an abstract structure that represents a set of objects together with the connections between these objects. The mathematical abstractions of the objects are called nodes (also corners) of the graph. The paired connections between nodes are called edges (sometimes also arcs). The edges can be directed or undirected. Graphs are often drawn vividly by representing the nodes with points and the edges with lines.
Vivid examples of graphs are a family tree or the subway network of a city (see illustrations). In a family tree, each node represents a family member and each edge is a connection between a parent and a child. In a metro network, each node represents a metro station and each edge represents a direct train connection between two stations.