Intelligent Design is a pseudo-scientific alternative to the Theory of Evolution that offers God as an explanation for the complexity of life while sort of pretending not to.
Proponents of Intelligent Design like to point to the concept of "Irreducible Complexity" as evidence that life was deliberately designed by an intelligent agent. Not only every living thing, but each cell in every living thing, is made up of a complex array of well-matched interacting parts that contribute to the basic function, such that the removal of any one of the parts causes the system to cease functioning. Commonly, an analogy is made to the ordinary mousetrap; if you remove any piece, be it the spring, the hook, the bar, or the base, it ceases to function as a mousetrap. It is "irreducibly complex," a set of events so unlikely to have occurred all at once by chance that we can state with assurance that it was the result of design. That, along with the brand name and the words "KILLS MICE" printed in big red letters on the base, constitutes strong empirical evidence for an intelligent mind behind the device. Any similarly complex organism in nature must also have been the product of intelligence.
Possibly our highest achievement.
Proponents of Intelligent Design are not only arguing from ignorance (I don't understand how it could have evolved, therefore it was designed) but are presenting a false dichotomy (if not evolution, then it must be a supernatural being). This obviously violates the principle of Occam's Razor, which says "given multiple, competing explanations for a given phenomenon, the correct explanation is probably the one with the fewest magic dingleberries attached."
In fact, ideas such as change of function, scaffolding, and improvement becoming necessity are easily adequate to explain how improvement by incremental steps could lead to an irreducibly complex system.
Proponents are also fond of drawing analogies between nature and the products of human design that depend on observing how radically different those two things are (google The Watchmaker Argument); this kind of defeats the purpose of analogies (that is, showing how two things are similar). The argument breaks down to this: nature looks like design, which we can recognize because it doesn't look like nature. In concrete terms, an eyeball is obviously designed because it works more like a watch than it does a pile of rocks. This bit of brilliance may be a genuine breakthrough in human thought; the next step, presumably, is to bang the rocks together.
So much for the Watchmaker Argument.
Proponents of Intelligent Design would like the "theory" taught in schools alongside evolution in the interests of "fairness." In reality, there is no controversy in scientific circles; a good example of a respected scientist who supports Intelligent Design is former Growing Pains star Kirk Cameron.
OK, it's a cheap shot, I know.
The supposed theory in fact offers no testable predictions, could not be proven wrong even if it was, and provides no supporting evidence not explainable without positing the supernatural. Which means it isn't actually science so much as religious philosophy with the occasional flowchart. It also leaves unanswered one important question: what the hell was God thinking here?