Reincarnation

The concept of death and rebirth as an animal or human depending on your actions in your past life is what is usually thought of when someone talks about reincarnation. But in Buddhism, that is not necessarily the case. Many historians believe that this idea was used during the time of Buddha in order to frighten people into acting better. The idea of reincarnation in
Buddhism centers around the "Law of Conservation of Moral Energy." This just means that any spiritual and mental action that is put out into the universe is not forgotten and will not go unnoticed. This idea then leads to the idea of karma, which is essential in reincarnation in Buddhism. Karma is the consequences of our actions and if we act morally in this life, according to Buddhist belief, in our next life we will be better off. In other words karma can be summarized as "a good cause, a good effect; a bad cause, a bad effect."

http://www.buddhanet.net/e-learning/karma.htm
http://www.buddhanet.net/e-learning/reincarnation.htm