The graphviz vocabulary provides an interface to your existing Graphviz installation, thus allowing you to create, edit, and render Graphviz graphs using Factor. For more information about Graphviz, see

This vocabulary provides the basic tools to construct Factor representations of graphs. For more details, see:
Graphviz data structures

Other vocabularies let you change a graph's look & feel, write cleaner code to represent it, and (of course) generate its Graphviz output:
Graphviz attributes
Graphviz notation
Rendering Graphviz output

After reading the above, you can see several examples in action:
Graphviz gallery