Network diagrams (also called Graphs) show interconnections between a set of entities. Each entity is represented by a Node (or vertice). Connection between nodes are represented through links (or edges). Networks is an entire field of research in itself. For example, you need to set up a layout algorythm that finds out an optimal position for each node. Several type of networks exist. They can be directed (flow) or undirected (connection). Links can be wheigthed or not.