In graph theory, which is an aspect of network science, a link between nodes can be either directed or undirected.

Whether a graph is directed or undirected depends on the nature of the connections between the nodes of a network. If the connections are directed, it means that one node points to the other in an unreciprocated fashion.

On a Personal Network like Twitter, for example, well-known people like celebrities and politicians have huge followings which they don't reciprocate. The flow of information is mostly one way - from the bigger, more central nodes to the smaller, more marginal nodes.

Contrast that with a Personal Utility Network like Facebook Messenger or WhatsApp, where connections are necessarily reciprocal. If you have a conversation with someone on Facebook Messenger, the flow of information and interaction is bidirectional. So Facebook Messenger and WhatsApp are examples of networks with undirected connections.

The direction of a link between nodes in a network is determined by which way, if any, the interaction between nodes in a network flows. That interaction can include the transfer of money, information, communication, and anything else that can pass between nodes as they interact.

A network that consists of directed links only is called a digraph, but true digraphs are rare. Usually, networks encompass a mixture of directed and undirected connections. Understanding the directionality of links in your network and mapping them visually leads to far better product design and prioritization of features.