Cook County Homeowners Exemption?

I am a first time home buyer and am in the beginning stages of looking for a home to purchase. When I am looking at homes online I see a big difference in annual taxes on homes in the same town. I know that the homeowners exemption is one that can make a big difference, but I am not too sure exactly what you can expect from it. When I do a search, I find the applications for it, but I am not able to find exactly what you are "exempt" from and if there is an expect percentage to save on the taxes. Thanks in advance for any help you can give!