There is a cycle of life, death, and rebirth. All humans who die will become Souls that will turn into Hollows or get sent to Soul Society. Souls/Shinigami that die will eventually become living Humans again. Souls and Hollows that are consumed by Hollows or die from any other cause than a Shinigami's Zanpakuto, such as the weapon of a Quincy, are destroyed and cease to exist, thus breaking the cycle for that particular Soul.