This is just a quick and dirty intro to Intelligent Design (ID).
Obviously, I think it'd be best for people to go straight to the source and read about ID directly from websites like Uncommon Descent and Evolution News & Views. To say nothing of the plethora of published works by ID theorists William Dembski, Stephen Meyer, Michael Behe, et al.
However, since I've had friends and others ask me to explain ID, I thought it'd be worth summarizing what I think is the main point of ID to them.
Francis Crick, who co-discovered the structure of DNA along with James Watson, once said:
Biologists must constantly keep in mind that what they see was not designed, but rather evolved.
By the same token, the militant atheist and evolutionist Richard Dawkins has said:
Biology is the study of complicated things that give the appearance of having been designed for a purpose.
As ID theorists have pointed out, the key word is "appearance." Individuals like Dawkins believe living organisms merely "appear" to have been designed when in fact they are anything but. They believe living organisms have instead come into being via unguided and purposeless natural processes. And that these natural processes are best explicated in neo-Darwinism.
By contrast, a reason ID theorists use the term "intelligent" in ID is because they wish to distinguish between an unguided and purposeless natural process vs a guided and purposeful process best explained by intelligent agency.
Now, virtually everyone recognizes there are many things in this universe which look like they're designed. Obviously we know things like computers, cars, and buildings have been engineered by humans. Likewise, various works of art, literature, music, movies, computer games. Similarly, we could say many inorganic materials like various plastics have been artificially designed. We could further include nanoparticles and arguably the synthetic elements in the Periodic Table too. Almost everyone including atheists like Dawkins would probably agree most if not all of these have been designed by intelligent agents i.e. humans.
But when it comes to living organisms, while everyone agrees life looks designed, atheists like Dawkins think actual design is an illusion. (I suppose in some ways similar to how some atheists think of consciousness.)
Worse, they practically become apoplectic if anyone so much as hints the design may not be an illusion but may in fact be actual design by an actual intelligence of some sort. That it may not have been unguided and purposeless after all. Or to put it another, if anyone casts doubt on the neo-Darwinian paradigm.
As an aside, it often seems as if it's all but a thought-crime to question Darwinism. This isn't hyperbolic language for effect, I don't think. There have been very real and unfortunate consequences. For example, many people's reputations have been unfairly marred. Many have lost their jobs and thus livelihoods and/or been blacklisted from future jobs due to their dissent from Darwinism. For starters, check out what happened to Richard Sternberg and Guillermo Gonzalez.
Getting back to the point, given living organisms appear designed, the next logical question should be: are living organisms, in fact, designed? Not: how or by what mechanisms have living organisms come to appear designed? Nor: who or what designed living organisms? (Dawkins, Coyne, Wolpert, and their kind react as equally irked by theistic evolution as they do by ID theory.) Yet neo-Darwinists often get ahead of themselves and confuse and/or conflate these and other questions and their related issues. They proceed, for example, to shout down ID theorists as Young Earth Creationists (YEC) in disguise, even though it's clear to anyone with a modicum of fair-mindedness and an ounce of familiarity with the movement that ID most decidedly is not. (Sometimes to the chagrin of many actual YECs!) Of course, these are good questions to ask, and questions which we should ask. But my immediate point is ID doesn't strictly speaking deal with these questions, not as their first port of call.
So, how can we tell if a living organism is truly designed? This is where intelligence comes in. Indeed, intelligence is what makes the crucial difference between the mere appearance of design vs actual design. In other words, there's a significant distinction between unintelligent design vs intelligent design, for unintelligent design means an unguided and purposeless natural process (i.e. neo-Darwinian theory), while intelligent design is, as I've already noted, the reverse.
Specifically, according to ID theorists, intelligence is a causal power that can arrange and adapt means to bring about teleological ends. This stands in distinction both to brute necessity which does not arrange or adapt means as well as to random chance which is not teleological or goal-oriented.
What's more, ID theorists have come up with ways to detect intelligence. I think Dembski's scheme is the most analytically rigorous. It'd be beyond the scope of this post to go into detail, but Dembski does point out three marks of intelligence: contingency, complexity, and specificity. By contingency he simply means if an object or event is unnecessary or, if you like, optional; if it occurred, even though it need not have occurred. Complexity refers to the fact that an object or event is difficult to reproduce by chance alone. And specificity is if an object or event exhibits an independent pattern. Dembski terms his theory of intelligence detection, specified complexity, and argues only intelligence can originate or generate specified complexity when prior to intelligence there was no specified complexity. Note this isn't the same as unintelligent natural processes making use of specified complexity.
If specified complexity is successful, then at a minimum it means we have sound and reasonable scientific and mathematical criteria to detect intelligent design in nature.