The Anthropic Principle is a loosely structured set of ideas which attempts to explain why certain observable features of the universe are the way that they are, and in particular why the values of particular universal constants seem to have been 'calibrated' so as to maximise the possibility of intelligent life coming into existence. The nub of the argument is that these constants could, in principle, have had any value and are effectively random. The fact that we see them as having these values is simply because we are here to see them. If the constants were otherwise, we would not exist, and there would be no observer to observe the values, and no questioner to pose the question.
Although they were not the first to suggest the idea, three separate versions of the Anthropic Principle were proposed by Barrow & Tipler (1986); these versions come in Weak, Strong and Final forms.
The Weak Anthropic Principle
In this version of the principle, Barrow & Tipler  suggest that the values of physical and cosmological quantities may be random, but are not equally probable; The values that they can adopt are governed by two separate requirements:
- That in the Universe there are places where carbon-based life can evolve
- That the Universe is old enough for it to have already done so.
In essence this simply says that we are here, therefore there must have been some way to get us here. We should not be surprised to find that any universal constants we observe would be configured in such a way as to allow for our existence.
The Strong Anthropic Principle
In this version of the Principle, Barrow & Tipler suggest that regardless of whether or not we are here to observe the fact, the Universe must (by necessity) have those properties which allow life to develop within it at some stage in its history.
In other words, in this principle, it is no coincidence that we are here; the Universe is expressly configured so as to enable and encourage the prospects of our existence.
The Final Anthropic Principle
In the most extreme version of the principle, Barrow & Tipler suggest that intelligent life (or something akin to it) is a necessary feature of the universe, and once it is created it can never be become extinct.
This version of the principle seems almost to imply that the sole purpose of the Universe is to create & sustain intelligent life; not only are the values of constants arranged to do this, but there seems almost to be some unseen force driving the Universe towards its goal.
The anthropic principle is commonly interpreted as evidence of an intelligent designer. Physicist Robin Collins said, "The extraordinary fine-tuning of the laws and constants of nature, their beauty, their discoverability, their intelligibility - all of this combines to make the God hypothesis the most reasonable choice we have. All other theories come short."  However, this is only evidence for ID if the strong/final versions of the anthropic principle are accepted, since the weak version makes sense without needing an intelligent designer.
Criticisms of the Anthropic Principle
In his web-page on The Anthropic Principle, the transhumanist philosopher Nick Bostrom suggests that the the Anthropic principle is only one manifestation of a much larger theory concerning self-location and observation. Within this he argues that the stronger versions of anthropic principles are simply confused, and while the weaker versions might be sound, they are in fact too weak to be useful. He argues that the current formulations of the principles are not formulated in any way as to yield observational consequences.