Cycle of Birth & Death
"The soul grows by reincarnation in bodies provided by nature, more complex, more powerful, as the soul unfolds greater and greater faculties. And so the soul climbs upward into the light eternal. And there is no fear for any child of man, for inevitably he climbs towards God." - Annie Besant
According to the Hindu sage Adi Shankaracharya, the world - as we ordinarily understand it - is like a dream: illusory. To be trapped in samsara (the cycle of birth and death) is a result of ignorance of the true nature of our existence.
It is ignorance (avidya) of one's true self that leads to ego-consciousness, grounding one in desire and a perpetual chain of reincarnation. The idea is intricately linked to action (karma), a concept first recorded in the Upanishads. Every action has a reaction and the force determines one's next incarnation. One is reborn through desire: a person desires to be born because he or she wants to enjoy a body, which can never bring deep, lasting happiness or peace (ānanda).
Reincarnation is also known as transmigration and metempsychosis.