The soul is eternal and can choose to have a spiritual body or material body. If one seeks material happiness he is awarded a material body; if one seeks spiritual happiness one's spiritual body remanifests.
An authoritative, comprehensive overview of the phenomenon called reincarnation, or "transmigration of the self" from one body to another, to another . . .
Originally published in Back to Godhead Magazine, here is a compressed, distilled version of the Bhagavad-gita—on a single Web page! (some scrolling required).