Original: A storm drain, storm sewer (U.S.), stormwater drain, drain (Australia and New Zealand) or drainage well system (UK) is designed to drain excess rain and ground water from paved streets, parking lots, sidewalks, and roofs.
Simplified: A storm drain, storm sewer (U.S.), stormwater drain (Australia and New Zealand) or surface water system (UK) is designed to drain extra rain and ground water from paved streets, parking lots, sidewalks, and roofs.