I occasionally get traffic from people asking that question, who find my previous entry, which mostly directed people elsewhere. I feel like elaborating in my own words, tonight.
Since there's a fair chance someone will click through a search with little previous experience of the term, woo, shortened from "woo-woo" is both a descriptive term for certain ideas as well as a person who holds them. A woo is someone who believes in woo. To get at the central concept, I'll define woo as the descriptive term for certain ideas.
"It is impossible to reason someone out of something that he did not reason himself into in the first place." -Jonathon Swift
That is a short description. Woo is pretty much anything a person believes without good reason. Science works in justifying knowledge with tight controls that isolate causes from confounding factors, consideration for coincidence, and intimate knowledge of all the failings us mortals can have. Woo, on the other hand, avoids all of that. I've seen a fair number of basic types of "bases" for woo:
1. Blind faith: For whatever reason, the woo has decided to believe in something knowing he has no justification for doing so. Often, it dips into "epistemological hedonism": He believes it because it makes him feel good, not because there's evidence for it. Common defense mechanisms that spring into action when these types of woos are questioned are appeals to consequences of belief (as opposed to the truth of the belief) and complaints that they can't cope with living in a world where the woo idea is wrong.
2. Ignorance of our failings: These people rely on anecdotes because they don't realize how unreliable our senses and memories can be, or various other ways how we can fool ourselves. That's one reason courts favor physical evidence over eyewitness testimony. Many of these people tend to be ignorant of statistical analysis and large numbers. Those who benefit often assume their experience is typical when it could be the result of being a lucky one. On the "alternative" "medicine" front, these people tend to think that the patient has an infallible sense of status: Feeling better one day means it works, and not that it might be the result of regression to the mean, faulty memory, being able to deal with pain, nausea, and other symptoms differently from the psychological aspects of the placebo effect. Defense mechanisms generally involve claiming special insight, pleas for us to always trust first impressions, that sloppily recorded personal experience trumps large, well documented studies, and generally assuming oneself to be incapable of being mistaken about their perceptions.
3. Deference to tradition and authorities: Trusting in what parents and authorities tell us was an important thing back in or primitive upbringing. The problem with that bit of our evolution is that it doesn't really do a good job of cleaning out bad ideas our ancestors pass on. Ideas should stand on their merits, not their age. Science tests ideas while woo is often an act of trust that those ancestors got it right somehow, through unspecified means.
There a probably more groups of fallacies that form the basis of various forms of woo, but those are three general types of woo foundations that come to mind. As for categories of stuff that falls under woo: Religion, alternative medicine, psychic powers, magic, giant conspiracy theories, alien visitation, cryptozoology, and a LOT of the "new age" stuff, which I like to pronounce to rhyme with "sewage." These things aren't woo because of any inherent nature of whatever they are. Magic, if it existed, wouldn't be any sort of opposite of science. We complain about people who claim magic exists, but don't do the footwork to test their claim.
Those who claim that something is beyond science like the "supernatural," "transcendental," or "divinity" don't understand the problem. Simply slapping a label on something doesn't change whether or not it has testable, observable effects, which is what science looks for.
What isn't woo: Emotions like love, concepts like justice and morality, abstractions like beauty and art. Love, consciousness, and those precious things that go on in our heads are physical. They cause physical effects like behavior, so they're physical. Justice, beauty, and so forth are ideas of what we find appealing. There's no scientific test for how ethical a course of action is because science doesn't ask how things ought to be, just how they are. Those ideas have a basis in our evolution and cultures, so we can know where those preferences come from, but not whether they're objectively good.
And to close up, I'll include a link to a long rant of mine where I complain how annoying and depressing I find woos to be.