How do you determine if you pay more taxes or get money back?

I know it is not for over half a year away but just out of curiousity I was wondering how do you determine if you get money back or have to pay. Like my fiance makes about 40,000 a year and has paid over 5000 so far to federal. I have heard that people who make under like 25,000 get most back but above that the amount you get back in return is less and less than what you pay in. Why is that and how is it figured? BTW we live in Ohio.