If the position is classed as non exempt and paid hourly, OT is required to be paid for every hour over 40 worked.  However, if the position is classed as Exempt, salary is paid and the employer can work you as many hours as they want.  I believe there is caviat at 60 hours within a given week though.  Below is a link to the US Dept of Labor Website.  This website is full of information that may be of value.

http://www.dol.gov/dol/topic/workhours/overtime.htm