Graph
Also known as: Network
A graph is a set of vertices plus a set of edges (undirected or directed, optionally weighted). It models social links, road maps, dependency relations, and state spaces.
Represent with an adjacency list (sparse, $O(V + E)$ space) or adjacency matrix (dense, $O(V^2)$). Core algorithms: BFS, DFS, shortest paths, MSTs, topological sort on DAGs.
Always state directed vs undirected and weighted vs unweighted before quoting complexities.