Hydroelectric Power

Taking the broadest view, the United States Department of Energy describes the derivation of power from water this way: "Water constantly moves through a vast global cycle, evaporating from lakes and oceans, forming clouds, precipitating as rain or snow, then flowing back down to the ocean. The energy of this water cycle, which is driven by the sun, can be tapped to produce electricity or for mechanical tasks like grinding grain."
In more conventional terms -- or at least those that relate to plugging in your toaster -- the process involves driving water, using a variety of methods, through turbines in order to generate electricity.
This can be accomplished in a variety of ways, from diversion to pumped storage. But by far the most common method for achieving hydropower is through building an impoundment facility, or dam, where water is backed up into a reservoir and then released through turbines as needed to meet energy demands