When the nodes in a graph have a strict parent-child relationship, the graph is a hierarchy. The hierarchy can have multiple levels: a parent node can have children nodes, which in turn can have grandchildren, and so on.
An example of a hierarchical network is the taxonomy system used in biology. The lowest levels in the hierarchy are the species, which are classified into the higher level genera, families, orders, classes, phyla and kingdoms.
A classical way to visualise taxonomical hierarchies is by using a dendrogram.
A dendrogram showing the taxonomy of the Felidae family, or cats. Source: made with RAWgraphs, Maarten Lambrechts CC BY SA 4.0
A dendrogram can also have a circular layout.
Source: made with RAWgraphs, Maarten Lambrechts CC BY SA 4.0
An alternative to dendrograms are nested circles (also called packed circles), in which each higher level node encircles its children nodes.
Source: made with RAWgraphs, Maarten Lambrechts CC BY SA 4.0