Design theory has been around for millennia, though it found its foothold in Christian thought because of the arguments of theologian William Paley. The basic concept of design theory is that things found in nature are so intricate and so perfectly formed to suit their purposes that they must have been designed by some pre-existing intelligence.
What are intelligent design and creationism and how are they related?
In 1802, Paley argued that if one were to look at a watch, with all its concise and intricate moving parts all working together to suit its purpose, one would certainly infer that there must have been a watchmaker. How then can someone look at, say, the human eye — infinitely more complex and intricate than a watch and so perfectly suited for seeing — and not infer that some intelligent being must have designed it?
Paley and his followers attempted to use design theory to ultimately prove the existence of a kind and just God by showing the balance and perfection of the natural world. But then Charles Darwin came along, pointing out the brutishness and imperfections found in the natural world. His theories of evolution soon supplanted design theory as the more popular.
Since the late 1980s, design theory has made a comeback of sorts. Creationism is, in its simplest form, the belief that the world was created by some supernatural being — normally the Christian God. Creationists fall into a wide spectrum of beliefs — from those who insist that the Bible's creation story is literally, word-for-word true, to those who finagle the concept of a "day" in the creation story to align better with scientific findings.
Intelligent design is one branch of creationist theory. Like William Paley, those who adhere to the intelligent design theory believe that evidence of God (or at least of a supernatural designer) can be seen in the complexity of life as well as in the perfect alignment of the laws of physics to allow life to form and flourish in the universe.
What makes intelligent design different from Paley's design theory is the acceptance of science into the equation, not just as a nuisance but as a tool to be used to strengthen the argument for intelligent design. Although most believers in intelligent design accept that Darwin's evolutionary theories aren't exactly wrong, they do believe that those theories eventually break down. At the point where his theories fail, the only plausible explanation of the complexity of nature is an Intelligent Designer.