The Fair Labor Standards Act is a Federal statute that, in essence, and put simply, requires that all workers of the 50 states and territories who are not "exempt" from being entitled to overtime pay (for work in excess of 40 hours in a given week) must get that pay. It's a long statute and there are a lot of twists and turns, but that, in essence, is it.
For a long time, the "big three" exemptions for wage and hour were: professionals, administrators, and executives. These terms, in essence, are fairly obvious. If you were any one of these three things (defined by the statute), then your employer could ask you to work more than 40 hours and not give you overtime. Most people who were properly one of these three things were probably on salary anyway.