Basic characteristics of graphs
A graph, 
, has nodes (vertices), 
, and edges, 
, but to uncover and analyze the rich structure present in a network, we’ll need to introduce additional concepts and terminology beyond just nodes and edges. Let’s start with those edges, which come in two flavors.
Undirected and directed edges
A key difference between our two real-world examples was that in our pizza example, the presence of an edge denoted a substitutability relationship that applied in both directions. An ACME Pizzas cheese and tomato pizza is considered by shoppers to be a substitute for a Premier Pizzas cheese and tomato pizza, and vice versa. In contrast, the arcs in our trade network in Figure 10.2 have a direction associated with them, indicated by the arrow at the end of each arc. The exports from the UK to Germany do not match the exports from Germany to the UK. In Figure 10.2, we represented this by having two directed edges between the UK and Germany nodes...
