If your employee is being paid per hour, then any time they spend working past the 40th hour in that week is considered overtime. According to the Fair Labor Standards Act (FLSA), the pay for overtime is one and half times the usual pay.
However, some workers may be exempt from overtime pay, meaning they don't receive it regardless of how many hours of they work. According to the FLSA, jobs that are exempt are those employed in "bona fide executive, administrative, or professional capacity (including any employee employed in the capacity of academic administrative personnel or teacher in elementary or secondary schools) or in the capacity of outside [salesperson]." Basically, if you're something like a CEO of a business, a restaurant owner, or a teacher, you are exempt from overtime.
For the most part, people who make yearly salaries as opposed to hourly wages are exempt too. This isn't always the case though, as those who don't make at least $455 a week still get paid overtime. Jobs that would be considered non-exempt would be cashiers or baggers at a grocery store, salespeople at a clothing store, dishwashers, or something of the sort.
The reason why some positions are exempt and others non-exempt has a lot to do with the fact that some jobs don't bring in a lot of money, If you work 40 hours a week but you get paid minimum wage, you're working a lot of hours but are going to have a difficult time getting by in life. Therefore, if you have to work even more than 40 hours, it's kind of like a bonus and an incentive to work.
On the other hand, those that are exempt usually are more financially secure because they are paid for the entire year, rather than by hour. As a result, they don't really get paid on how much they work, but by what they get accomplished. For example, someone running a business wouldn't be judged by how many hours they worked, but more of how much money they are bringing to their company.