Learn in the textbookBasics of Graph Theory
DefinitionA directed graph (有向グラフ) is a graph whose edges have a direction and are drawn as arrows.
ExplanationAn arrow shows a relationship running from the vertex at its tail to the vertex at its head, and the reverse relationship does not hold unless there is a separate arrow for it. Directed graphs represent things like the order of tasks, links between web pages, and road networks made of one-way streets.
AnalogyThink of a road network with one-way signs. You can only go the way the signs point, and getting back takes a different road.
Memory hookDirected literally means having a direction. Check whether the line ends in an arrowhead.