NodeXL is an Excel template that displays and analyzes network graphs, or graphs for short. A network graph is a collection of points connected by lines. The points are called vertices or nodes, and the lines are called edges. NodeXL always uses the term "vertex" rather than "node," contrary to its product name.
Here is a simple network graph:
Network graphs can be used to model a wide variety of real-world and theoretical systems, ranging from social organizations consisting of people connected by relationships, to cities connected by transportation routes, to atoms connected by chemical bonds.
If you are new to network graphs, the Wikipedia article Graph Theory can serve as a good starting point for learning about them.