Gregory XII is the real pope, Benedict XIII and Alexander V are anti-popes.
Pope Gregory XII succeeded Pope Innocent VII on November 30, 1406 until July 4, 1415
Benedict XIII (September 28, 1394 - May 23, 1423), known as el Papa Luna in Aragonese and Spanish, was an Aragonese, and is officially considered by the Roman Catholic Church to be an Antipope.
Alexander V reigned from June 26, 1409 to his death in 1410 and is officially regarded by the Roman Catholic Church as an antipope. He was succeeded by Baldassare Cossa (the above answer implies that there were four popes at one time). The popes were elected in Pisa.
It is usually called the "Western Schism" while the term "Great Schism" is more often applied to the East-West Schism of 1054.
