A dam is a structure built across a river or stream to retain water. Dams have several purposes including water supply, flood control, irrigation, hydroelectric power generation, navigation, and recreation. Dams are classified based on their structure as arch dams, gravity dams, buttress dams, or embankment dams. While dams provide many benefits, they can also fail due to construction or geological issues, causing downstream flooding. Dams also have disadvantages like displacing communities and damaging natural environments.