An attempt by the apologists for creationism to artificially inject their metaphysical theory with scientific credibility. Intelligent design makes the "deduction" that the development of complex life on this planet was an intelligent creator, largly on the basis of argument by personal incredulity, a flawed understanding of Darwinian natural selection and fundamental misunderstandings of basic principles of thermodynamics and biology. Advocates claim natural selection/evolution (which they frequently confuse) are impossible, and often have links to other biblical-literlists that believe the earth is only 30000 years old, and other such fantasies. There is reason that the evidence would lead any impartial person to come to such conclusions; it is therefore an example of wishful thinking and by the principle of Occam's razon is a metaphysical (faith) based belief, not a scientific one. The continued deliberate obtuseness on this issue has led to fundamentalists attempting to force science teachers to present it in the classroom, and the general devaluing of science in the population of the US and other countries as a whole.
Although there is not a scrap of evidence for intelligent design, it is impossible to disprove (because god can do anything!), and hence it is a metaphysical theory.
Because natural selection is demonstrably observably occuring right now, it is a scientific theory.