The basic premise is simple. Pour water into media that absorbs water and is able to have air pulled through it. On the other side your airstream is moisture laden, when it hits the skin the evaporation causes a cooling effect on a person.
How Much..Cooling..?? Depends on humidity levels but anywhere from 10-15 degrees cooler than the ambient air.
Applications are big open areas that don't mind moisture put into air...Livestock/Barns, open warehouses, outdoor events etc. PLUS, they are usually less expensive that air conditioners and require just a plug in for electrical needs.
Take a look at the video and consider an evaporative cooler. It might help keep your cool this Summer.