A concept which essentially states that the world we know is not the only realm in which one can perceive events and exist.
Buddhists believe that mortal life is characterized by an eternal cycle of life and death, and that the true goal of consciousness is to move beyond it, into enlightenment and unity. The path off the circular road is paved with ascetic denial of the flesh and introspection into the nature of reality.
Christians believe that mortal life transpires in the central plane of the universe, and that above that plane is Heaven, a place of supreme bliss and reward, and below it is Hell, a place of eternal pain and regret. Entrance to one or the other is determined after death, based on one's deeds.
Of course, this is not true for all sects of these religions, and many would strenuously disagree with these concepts. I simply state them as examples.