It is often argued that the eye is so complex that it could not have evolved naturally. The idea being that an eye with only "half" the parts in place would not work, therefor evolution would have never favored it and the more complex human eye we have would never have evolved.
However, the evolution of the eye has been studied extensively, and its history is more-or-less well established. First, there were light-sensitive cells which merely indicated which way was the Sun. A slight indentation makes a sense of direction possible. Mucus in the pit focuses the light. If the mucus hardens, you have a proper lens, and so on.
In fact, every stage of the eye's development is still around on Earth today. A snail's eye is less than "half" a human eye, yet it serves the snail well enough to help its survival.
Please watch this video which beautifully describes the evolution of the eye for a more visual example.
[ References: Wikipedia - Evolution of the Eye ]
Intelligent Design (ID) is the hypothesis that a higher intelligence (ex: a god, space aliens, time travellers from the future, etc.) guided the course of evolution. Most often, Intelligent Design refers to a god, typically the Christian/Jewish/Muslim god, YHWH or Allah. In other words, God from the Bible.
ID differs from traditional religions in that proponents recognize that biological evolution is responsible for the development and diversity of life on Earth. It is similar to traditional religions in that it typically posits a supernatural explanation for what guided the evolution of certain species (including humans). In this way, ID is similar to the 4th century BC belief put forward by Plato and Aristotle of a "prime mover," that is, a being which set the natural world in motion.
This has been a hotly debated question in the media, classrooms, and many churches. According to authors like Michael Behe, there are certain structures in nature which he terms "irreducibly complex," which, he asserts, could not have evolved naturally. He gives bacterial flagellum and the eye, among others, as examples. If true, these might be evidence for Intelligent Design.
However, the vast majority of the scientific community regards Intelligent Design as psuedoscience or "junk science." The structures that Behe and others give as examples have proven evolutionary histories. For an example, watch The Evolution of the Eye video on this site. Recently at the famous Dover trial, the court held that "Professor Behe's claim for irreducible complexity has been refuted in peer-reviewed research papers and has been rejected by the scientific community at large."
To date, there has never been any evidence, agreed upon by the scientific community, to be proof of Intelligent Design.
This is a statement often asserted by proponents of Intelligent Design, but it is simply not true. The argument goes that since the flagellum (the tail-like structure many bacteria have which helps them "swim") is a very complex machine, it could not have evolved naturally. This is because if you remove any of the proteins that make up the flagellum's motor, it can no longer function.
However, as this video beautifully demonstrates, the flagellum does function with fewer proteins, just not as a flagellum. Every part of the flagellum is made of proteins already in the cell, performing other functions. This is one of the most interesting aspects of Evolution: that adaptations can change roles, or combine with other adaptations to perform new roles.