Edges either point one way or go both ways, and that choice is the whole distinction. In an undirected graph an edge means a mutual relationship, like two friends. In a directed graph an edge goes from one vertex to another, like a one-way street or a follow.
The practical effect shows up in reachability. Undirected edges let you travel from either end to the other. Directed edges only let you move along the arrow, so A reaching B never guarantees B reaches A.
Pick based on whether your relationship is symmetric.
- Undirected: friendship, physical roads that run both ways, network cables
- Directed: follows, task ordering, web links, dependencies
Getting this wrong quietly breaks logic. If you model "depends on" as undirected, a topological sort becomes meaningless, and cycle checks stop making sense. Match the edge type to the real relationship first.
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