Whether something is a conspiracy theory or not has no bearing on whether it's actually true.
There have been actual conspiracies in the past. However usually we can only ever tell that it was a conspiracy in
hindsight after all the evidence is out.
As far as I can tell, most of the circulated, ongoing conspiracy theories usually contradict the the available evidence (ie. they're demonstrably false) or they at least do not have enough credible evidence behind them to reasonably tell that they're true (they're "not true" in that sense).
Someone implied to me that 9/11 might not have been a terrorist attack and not by Al-Quaeda. I found that insinuation absurd, so I looked it up and indeed there is not a single piece of credible evidence in its favor, but there's a mountain of evidence against that idea.
A senior student actually held a speech in front of some of the freshmen students (he was in charge of welcoming us) and he talked about the dangers of vaccines and outright told everyone to not get vaccinated, if possible. I was very skeptical about his claims, but I didn't know enough about the subject at the time to neither agree nor disagree. When I looked vaccinations up later, it turned out that he was full of bullshit. I think it's outrageous that they let him spread such misinformation.
Someone else once put to me that the Bilderberg Group [http://en.wikipedia.org/wiki/Bilderberg_Group] conference, an annual meeting of world leaders, was a very suspicious, because it's secret, implying that they plan nefarious stuff there. Of course there was no evidence again. And without evidence you cannot make any honest claims.
I looked it up and the secrecy has the purpose of ensuring that the people speaking there will give their honest opinions.
Never attribute to malice that which is adequately explained by stupidity.
-Hanlon's razor
By the way, another thing to be aware of is that there are two definitions of the word "theory [http://en.wikipedia.org/wiki/Theory_(disambiguation)]" that are often conflated:
1) The colloquial meaning; a speculative guess.
2) The scientific meaning; a principle or body of principles for explaining observed facts or phenomena, such as cell theory and the theory of gravity.