Since some background knowledge of rebirth and karma is useful for understanding Buddhism, there now follows a brief introduction to these topics taken from Geshe Kelsang's book, Eight Steps to Happiness:
The mind is neither physical, nor a by-product of purely physical processes, but a formless continuum that is a separate entity from the body. When the body disintegrates at death, the mind does not cease. Although our superficial conscious mind ceases, it does so by dissolving into a deeper level of consciousness, call 'the very subtle mind'.
The continuum of our very subtle mind has no beginning and no end, and it is this mind which, when completely purified, transforms into the omniscient mind of a Buddha.
Every action we perform leaves an imprint, or potential, on our very subtle mind, and each karmic potential eventually gives rise to its own effect. Our mind is like a field, and performing actions is like sowing seeds in that field. Positive or virtuous actions sow the seeds of future happiness, and negative or non-virtuous actions sow the seeds of future suffering.
This definite relationship between actions and their effects - virtue causing happiness and non-virtue causing suffering - is know as the 'law of karma'. An understanding of the law of karma is the basis of Buddhist morality.
After we die our very subtle mind leaves our body and enters the intermediate state, or 'bardo' in Tibetan. In this subtle dream-like state we experience many different visions that arise from the karmic potentials that were activated at the time of our death. These visions may be pleasant or terrifying depending on the karma that ripens. Once these karmic seeds have fully ripened they impel us to take rebirth without choice.
It is important to understand that as ordinary samsaric beings we do not choose our rebirth but are reborn solely in accordance with our karma. If good karma ripens we are reborn in a fortunate state, either as a human or a god, but if negative karma ripens we are reborn in a lower state, as an animal, a hungry ghost, or a hell being.
It is as if we are blown to our future lives by the winds of our karma, sometimes ending up in higher rebirths, sometimes in lower rebirths.
This uninterrupted cycle of death and rebirth without choice is called 'cyclic existence', or 'samsara' in Sanskrit. Samsara is like a Ferris wheel, sometimes taking us up into the three fortunate realms, sometimes down into the three lower realms.
The driving force of the wheel of samsara is our contaminated actions motivated by delusions, and the hub of the wheel is self-grasping ignorance. For as long as we remain on this wheel we shall experience an unceasing cycle of suffering and dissatisfaction, and we shall have no opportunity to experience pure, lasting happiness.
By practicing the Buddhist path to liberation and enlightenment, however, we can destroy self-grasping, thereby liberating ourself from the cycle of uncontrolled rebirth and attaining a state of perfect peace and freedom. We shall then be in a position to help others to do the same.