Sa?s?ra is the beginning-less cycle of repeated birth, mundane existence, pain, suffering and dying again perpetuated by desire and ignorance due to your own karma. Rebirths occur in six realms of existence: Naraka the Hell Realm, the Realm of Hungry Ghosts, the Animal Kingdom, the Human Realm, the Realm of Constant Warfare and lastly the Realm of Demi-Gods. Samsara only ends if a person attains nirvana and breaks free of their own desires and becomes self-less. -Wikipedia.org