(definitions pilfered -- with slight modifications, perhaps -- from www.cs.usask.ca/resources/tutorials/csconcepts/graphs/tutorial/definitions.html -- now a dead link).
graph: consists of a set of nodes (or
vertices), a set of arcs (or edges), and a function that associates a
pair of vertices with each edge
transport network: a connected, directed graph (with no self
loops) that has only one vertex
of indegree zero (the source), and one vertex of
outdegree zero (the sink).
Each edge is associated to a number which represents the limit to the rate
of transportation of the product (called the capacity).
tree: a connected graph in which there is
only one path connecting each pair of vertices