A conspiracy theory is a paranoia-induced collection of explanations that are created by usually anti-government or anti-authoritarian wackos that want the world to conform to their fictional universe.
In most cases, these "theories" are intended to make a large-scale event become an attack by the "grand cabal of evil men trying to rule the world from the shadows". If there is an important event in history, expect one of these contrived theories to appear as an alternate explanation for the simpler, real reason for the event. Remember, it doesn't matter to them if there is a real conspiracy behind the event that is openly visible, it has to be their conspiracy behind it.
Conspiracy Theorists in general have the attitude that they are the only smart people who can actually see the world for what it really is. It doesn't matter to a conspiracy theorist if their explanations contradict each other, since only the end result matters to them. They also tend to toss as many claims out as possible and repeat themselves constantly, making refuting them extremely difficult. If you fail to address even one of their claims (even if that happens to be a repeat of one that you've already addressed, but worded differently), they will latch onto that to dismiss the rest of your argument.