It depends.
At one person I know company, if they worked for 10 hours a week, they still have to be paid for all 40, and if they came in atleast 1 hour each day, they don't lose vacation. At another person I know they have to work atleast 40. But at the person who can work under 40, they don't get overtime if they work over 40. The person who must work atleast 40 hours, does get overtime at over 40 hours. The person who can work any number of hours, each year the company shuts down for 2 weeks during december, hourly people do NOT get paid. She still must be paid.