A topological sort of a dag (directed acyclic graph) $G = (V, E)$ is a linear ordering of all its vertices such that $if$ G contains an edge $(v, w)$, then $v$ appears before $w$ in the ordering. In other words, it is the ordering of the vertices that respects the direction of the edges. (If the graph contains a cycle, then no linear ordering is possible.)

For example, we can imagine using topological sort to plan a course schedule. (Although it would be difficult to graduate on time if we could only take one class at a time.)

The algorithm is quite simple and uses post-order depth-first search.