Intelligent design is what's being framed these days as an alternate theory of how life on Earth today came to be. The argument, such as it is, asserts that since Darwin isn't perfect, we ought to teach other ideas as well.
What these ideas are isn't clear. They could be anything from the Church of Scientology's claim that life on Earth was planted by aliens, to the notion that the Christian God cooked it all up. The latter seems to be the one the intelligent designers are pushing. But the fact that intelligent design or creationism has gained any ground at all is based on a fundamental misunderstanding as to what constitutes a scientific theory.
There are three ways in which we use the word "theory." Two are casual. I might get up in the morning, find the coffee's all gone, then theorize that someone drank it. But what I'm really doing is guessing based on past experience.
Television shows and detective novels also throw around the word "theory." A detective might say, "Our working theory is that Tom Smith is the crook," based on whatever facts he's discovered. But again, what we're talking about here is not really a theory, but, depending on how much investigative work has been done, an idea or an educated guess.
This casual use of the word "theory" has so blurred its true meaning in the public mind that it has lost its special status. But a scientific theory is not a guess, educated or otherwise. A scientific theory is a machine that produces sensible explanations.
If I throw a baseball and wonder why it falls to earth, there's a theory called gravitation to explain it. Gravitation has been worked out over centuries starting with Newton, reaching its pinnacle with Einstein and continuing today in research institutes all over the world. The reason for this is neither that it's a cute idea, nor that it fits in with someone's religious ideology, but because it works. Gravitation in its present form can explain not only what, given a certain bunch of circumstances, happened, but what, given the same set of circumstances, will happen.
If a Pacific volcano erupts, the theory of plate tectonics has the capacity to do the same thing. No, it's not perfect. We still have a devil of a time predicting earthquakes, but as more and more observations and experimental results come in, we're getting much better with volcanoes and predicting movements of the earth's crust.
Neither gravitation nor plate tectonics are ideas or guesses, but coherent logical frameworks that explain bodies of observational evidence. Their results have been tested over and over again. Gravitation has been tested to the extent that we don't even call it a theory anymore, but a law. Are there anomalies, things gravitation can't explain? Absolutely. But should anyone suggest this is a good reason why the medieval notion that the planets stay up because God's holding them up must be taught alongside gravitation, they'd earn a one way ticket to the loony bin.
The whole evolution/creationism argument makes me think of the famous cartoon by Sidney Harris. Two scientists are standing at a blackboard covered with sets of calculations, one of them points out the fact that the other has scribbled the words, "then a miracle occurs" in the middle of the board. The other guy tells him, "I think you should be more explicit in step two." This is funny precisely because this isn't the way you do science. It's the way you do religion.
There's an Australian bearded dragon sitting on my computer monitor. Her name is Juliet, and she likes the computer because it's warm. Her boyfriend, Romeo, is sleeping on the patio beneath an inverted pot. He probably won't stir until the sun heats his abode. Lizards are simple that way: heat-activated.
This I know about lizards, and a few more things as well, but there are all kinds of things I haven't got a clue about. If I wanted to learn, the theory of evolution could provide sensible answers. Intelligent design, on the other hand, couldn't provide anything. In short, if my kid asks his teacher why lizards like to sit on computers, "because a miracle occurs" is a lousy answer.