It would depends on where one works. My (20 year old) daughter works for a survey company. She has regular hours which she mostly works. But like last Friday they sent her (well everyone) home about 3 hours early. She doesn't get paid for that.
According to the FLSA, if an employee is scheduled to work but is sent home due to no work, are entitled to some kind of pay. For more information about the FLSA laws you can visit the website http://www.dol.gov/compliance/laws/comp-flsa.htm