They are basically giant hot tubs. Cooling water can be pretty much anything. In most cases, it's river water. The water goes into the intake, where there are basically just screens to catch any debris and even fish. This water goes into the tubes of the condenser. The steam from the turbine, hits these cooling tubes, and condenses the steam back into boiler water. The boiler water then goes into the "hotwell," which is then returned right back into the boiler to make steam. Rinse and repeat.

The cooling water though, is now super hot. This water is sent out to the cooling towers. At a nuclear plant, those are those big ass signature towers. At a coal plant, they are typically rows of fans. Depending on the power plant, the water is then returned to the river, or rerouted back into the power plant as cooled water for re-use