The document discusses depth-first search (DFS) graph algorithms. DFS explores a graph by going as deep as possible at each step and backtracking when no further progress can be made. It distinguishes between different types of edges discovered: tree edges connect newly found nodes, back edges connect to ancestors, and cross/forward edges connect between subtrees or ancestors and descendants. DFS runs in O(V+E) time and can detect cycles in O(V) time by checking for back edges.