What is topological sorting and how do you implement it?

Topological sort orders vertices of a directed acyclic graph (DAG) such that for every edge u->v, u comes before v. Two approaches: Kahn's algorithm uses BFS with in-degree tracking, repeatedly removing zero in-degree vertices. DFS-based approach recursively visits all descendants before adding current node to result (then reverse). Both are O(V+E). Used for task scheduling, build systems, and course prerequisites.