In mathematics, networks are called graphs, nodes are called vertices, and links are called edges. In computer science (and in the D3 libraries) these terms are often used interchangeably. You don't have to be fluent in graph theory to create network visualizations, but it's good to know the basic terms and concepts, since it will help you choose the best network layout for your data.
A directed graph or network is one where each connection between two nodes has a specific direction (moving from A to B is not the same as moving from B to A). In an undirected (or symmetrical) graph, there is no difference. For example, a graph showing flight connections between countries is directed, since the number of flights leaving a country is not necessarily equal to the number of flights entering it. But a graph where the connections between countries...