Cooling towers are used to cool the hot steam as it rises in the cooling tower. Most cooling towers are open on all sides with slates that allow air to enter and mix with the sprayed/evaporated water to it. As the water evaporates it cools in the tower, falls down (falling water is known as rain), and is recycled in the system again to extract more heat to provide more energy. The results looks kind of like this:
Photo courtesy of Google: Cooling Tower System
However, not all cooling systems are these "square" or "round" shapes. There are some that look slightly more familiar if you've seen The Simpsons:
Courtesy of Google: Hyperbolic cooling tower
Hyperbolic cooling towers perform the same function and are more expensive to build and more difficult to seismically qualify that round cooling towers. These towers are for plants that cannot be cooled by "natural" means (I'll get to this momentarily). The advantage of hyperbolic towers is that all air enters at the bottom of the cooling tower (not through the sides) and passes up through the entire tower height producing a "chimney" effect that strengthens the draft, promoting better heat rejection. The reduced diameter would increase air velocity to counter the downward pressure of the water drops.
Generally, cooling towers are only used by plants that cannot cool the plant by "natural" means. This means that if a plant is located on a lake, ocean-side or river, those bodies of water can be circulated through the system to cool the reactor and then expelled out again. This may sound dangerous or bad for the environment, but it really isn't. The water emerges from the plant only a little hotter than when it went in. The water never actually comes into contact with anything contaminated in the plant. It is in a closed system that simply cools the system, as is shown below. There is no risk of contaminants being released into the ocean.
Photo Courtesy of PG&E (read article on Once Through Cooling here)
