The article Jez posted was enlightening from a psychological standpoint, and there was a lot of good discussion generated in that string, so I won't comment on any of that except to say that the bias of the writer is obvious regarding "conspiracies".
Here I just want to post a link to the 1967 CIA document that pretty much coined the term "Conspiracy theory" with detailed instructions how it was to be inserted into the media and used to deride any dangerous inquiries into the JFK killing or anything else they wanted to keep hidden. If you've never seen it, it's most instructive and explains a lot of what has happened in the media and in our collective psyches since. You'll have to admit that it's one of the most successful mass perception-management devices ever created. Not only has it come to be used almost universally to cue us all in to what we're not to inquire or even think about in a serious way, triggering an automatic response (think Pavlov) of dismissal or even derision of the argument/evidence or person presenting the argument/evidence thus labeled, but nearly everyone uses it, and often people use it as a kind of self-protective introduction to something controversial they're about to talk about: "I'm not a conspiracy theorist, but..." It works very much like the terms "antisemite" and "UFO nut" to attack the critical thinker's argument or him/her directly to turn other people's critical thinking off. ...Wow, what a lot of words to talk about something that we pretty much all know.
So, without further ado, here's the fascinating CIA document: http://www.jfklancer.com/CIA.html