The federal Fair Labor Standards Act states that employees can't work more than a 40-hour week without getting time-and-a-half for overtime. Some employees are exempt from the law; they can work 45, 50 or 60 hours a week without any right to time-and-a-half. Federal law says that to exempt you, your employer must pay you a minimum amount each week even if you don't work your full schedule.
If you're actually paid on an hourly basis -- you get less money if you work fewer hours -- you are usually not an exempt employee and you're entitled to overtime. If your pay is merely expressed in hourly terms, you could still be exempt; it's a common feature of payroll-calculating software, even for a salaried worker. The test is whether you have a guaranteed minimum pay in any week that you work. If you're suspended or take unpaid personal leave, your employer can dock your pay without affecting your exempt status.
Some hourly employees are an exception to the rules, because they're in jobs the Fair Labor Standards Act doesn't cover. Truck drivers and railroad workers, for example, are covered by other federal laws. The act specifically excludes some jobs, such as movie-theater employees and agricultural workers. The law identifies other jobs as exempt by definition; outside salespeople -- as opposed to salespeople working in stores -- count as exempt regardless of how they're paid, for example.
If you're a salaried, exempt worker, you're entitled to come in late or leave early without losing any pay. Your company can require that you punch a time clock, even though you're on salary. It also can require you use paid time off, if you have any, to cover your absences, but if you have no PTO, the company can't dock your income. You can be exempt even if your company pays you extra if you put in more time, provided that you still have a guaranteed baseline salary.
If you believe you're illegally classified as exempt, you can contact the United States Department of Labor. If the department finds that your employer misclassified you, it can award you up to two years of unpaid overtime. Even if you're paid on salary, you're still nonexempt if your pay falls below a federal minimum amount ($455 per week as of May 2011), or if the department classifies your job duties as administrative, executive or professional. It's the actual job duties, not the title placed on them, that count.