A N T H R O P I C  P R I N C I P L E

Weak Anthropic Principle (WAP): The observed values of all physical and cosmological quantities are not equally probable but they take on values restricted by the requirement that there exist sites where carbon-based life can evolve and by the requirements that the Universe be old enough for it to have already done so.

Strong Anthropic Principle (SAP): The Universe must have those properties which allow life to develop within it at some stage in its history. Because:

  1. There exists one possible Universe 'designed' with the goal of generating and sustaining 'observers'. Or...
  2. Observers are necessary to bring the Universe into being (Wheeler's Participatory Anthropic Principle (PAP)). Or...
  3. An ensemble of other different universes is necessary for the existence of our Universe (which may be related to the Many_Worlds interpretation of quantum mechanics).
Final Anthropic Principle (FAP): Intelligent information-processing must come into existence in the Universe, and, once it comes into existence, it will never die out.
R. Michael Perry has another interesting variant of the anthropic principle:

Individual Anthropic Metaprinciple (IAM): The universe that I as an observer perceive is so structured that I am immortal.

A very interesting and detailed introduction to Anthropic Principle

A Related and Interesting Article by Victor Stenger:  Cosmythology 

SUGGESTED READINGS

[1] John D Barrow & Frank J Tipler - The Anthropic Cosmological Principle
(OUP 1986 ISBN 0-19-282147-4 pp706) Excellent, heavily mathematical but readable. Somewhat wacky conclusions. For more wackyness, see this review of Tipler's Physics_of_Immortality.
[2] John D Barrow - The Origin of the Universe
(Science Masters series ISBN 0-297-81497-4 pp150) Excellent.
[3] Paul Davies - The Mind of God
(Penguin 1993 ISBN 0-14-015815-4 pp254)
[4] Paul Davies - Are We Alone?
(Penguin 1995 ISBN 0-14-024585-5 pp108)
[5] Paul Davies - The Accidental Universe
(CUP 1982 ISBN 0-521-24212-6 pp139) Very good. Mathematical, a little dated now, but much here that can't easily be found elsewhere.
[6] Paul Davies - The Last Three Minutes
(Science Masters series 1994 ISBN 0-297-81502-4 pp162)
[7] John Gribbin - In The Beginning
(Penguin 1994 ISBN 0-14-017792-2 pp274) Excellent, very readable. Also has curiously wacky conclusions. But then, don't we all. Also see Gribbin's home_page.
[8] John Gribbin & Martin Rees - Cosmic Coincidences
(Black Swan 1991 ISBN 0-552-99443-X pp302)