A DAG (Directed Acyclic Graph) is a graph of nodes connected by directed edges with no cycles — you can never loop back to where you started. It is the model for anything with dependencies that must run in order: data pipelines, build systems, task schedulers, even spreadsheet recalculation.
Worked example: an Airflow pipeline is a DAG — “extract” must finish before “transform,” which must finish before “load”; the scheduler runs independent branches in parallel and respects the edges. Gotcha: the acyclic property is what makes the order well-defined (a topological sort exists) — introduce a cycle (A depends on B depends on A) and there is no valid order, exactly the error a pipeline or build tool throws; DAGs also make it easy to re-run just the downstream of a changed node.