Intelligent Design is the concept that a supernatural designer operated at some time in the past, or operates today, in our natural world and leaves detectable evidence of that influence.
In short, Intelligent Design claims to be an improvement or alternative to evolution. Intelligent Design claims that evolution is not a complete explanation for all of the varied forms of life we see on earth today. The complete explanation requires the periodic intervention, by supernatural means, in the evolutionary history of living things on this planet. In other words, miracles have occurred in the past and can be detected today.
This is my nutshell definition of Intelligent Design. It is the best you will find, because Intelligent design proponents don't specifically define "Intelligent Design." For example, if you go to the website of the Discovery Institute, the recognized leader in promoting Intelligent Design, the FAQ defines Intelligent Design vaguely:
The theory of intelligent design holds that certain features of the universe and of living things are best explained by an intelligent cause, not an undirected process such as natural selection.
The , features, scope, specifics, and practical applications of the of the concept, are all missing. Most significantly, the Discovery institute and Intelligent design proponents profess to be curiously unconcerned about the identity of the designing entities. You will, however, find two articles linking evolution to Hitler, but that's another matter. The Discovery Institute site is not a good place to start if you want to find out what Intelligent Design is actually about.
If you are already familiar with the concept, an article from Michel Behe at the site offers a summary without using the term "Intelligent Design:"
First, he quotes Charles Darwin from Origin of the Species:
If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down.
He then notes:
A system which meets Darwin's criterion is one which exhibits irreducible complexity. By irreducible complexity I mean a single system which is composed of several interacting parts that contribute to the basic function, and where the removal of any one of the parts causes the system to effectively cease functioning. An irreducibly complex system cannot be produced directly by slight, successive modifications of a precursor system, since any precursor to an irreducibly complex system is by definition nonfunctional.
Since natural selection requires a function to select, an irreducibly complex biological system, if there is such a thing, would have to arise as an integrated unit for natural selection to have anything to act on. It is almost universally conceded that such a sudden event would be irreconcilable with the gradualism Darwin envisioned. At this point, however, "irreducibly complex" is just a term, whose power resides mostly in its definition. We must now ask if any real thing is in fact irreducibly complex, and, if so, then are any irreducibly complex things also biological systems?
This is the foundation for Intelligent design--that there are some natural organic systems which could not have not evolved but which had to be specially created. Created by who? Well, we don't know. All we know is that we humans didn't do it. The key to understanding Intelligent Design is that it teaches that evolution is not the answer to the biological diversity we see around us. That diversity only came out by the occurrence of miracles. Intelligent Design boldly promises that we can reliably detect miracles--supernatural interventions in everyday life. How well does Intelligent Design deliver on this promise? We'll see.
Michael Behe's article is condensed from his book, Darwin's Back Box. This book, written in 1996, is the only attempt I am aware of the apply "intelligent Design" concepts to specific living things. He has not written anything else since 1996 attempting to identify any biological systems that may be irreducibly complex or are "intelligently designed."
You will also note that, unlike the Discovery Institute, I will link to all texts, articles and treatises, whether I agree with them or not, if a link is available and there is no objection. Unlike the Discovery Institute, I am not afraid of the truth. I challenge the Discovery Institute to link to the same texts and websites I do. In return, I will link to any that they suggest.