A Swamp Cooler is a Poor-Man's Air Conditioner:
If you drive around and see a big louvered box sitting on people's roof; that's a Swamp Cooler.
I uses a mist of water to cool the air that is being blown into the house.
It is better than nothing.
It was installed in a lot of houses in the 60's and there are still a lot of them, because people cannot afford to replace them with A/C units. (All the ducting would have to be re-done too.
With the new, energy efficient A/C units, the difference is probably very little, but the results are world's-apart.