In graph theory, a Flow network is a directed graph where each edge has a capacity and each edge receives a Flow. The amount of Flow on an edge can not exceed the capacity of the edge. Often in Operations Research, a directed graph is called a network, the vertices are called nodes and the edges are called arcs. A Flow must satisfy the restriction that the amount of Flow into a node equals the amount of Flow out of it, except when it is a source, which has more outgoing Flow, or sink, which has more incoming Flow. A network can be used to model traffic in a road system, fluids in pipes, currents in an electrical circuit, or anything similar in which something travels through a network of nodes.   Wikipedia