Power plants use large amounts of water (and thus are sited next to rivers or lakes) that is super-heated (with coal or uranium or natural gas) to steam.  The steam turns a turbine, which is attached to a generator, and the resulting electricity is then transferred to end users.  The water is condensed and recycled in these cooling towers, which also vent excess steam into the atmosphere.  The large hyperboloid (a shape that produces a natural upward draft) cooling towers at nuclear power plants serve the same function as these smaller units.