Today I'm going to tell you about the Argument from Authority fallacy, what it is, how to recognize it, and why it's a fallacy.
Authority is a useful shortcut in arguing, because it's a cogent sign of expertise, and implies correctness. When a nuclear physicist tells me that all atoms of the same type and isotope-ness have the same weight, I can trust him on this being true, because his expertise has assured me that he has studied about this and isn't just making things up. Even if I don't believe him, verification will only take me tons and tons of time.
However, the fallacy occurs when experts attempt to argue outside of their domain. The nuclear physicist from my example above is no more an expert on, say, Economics, than I am, and if he argues that he is, he's hoping that people will assume that his one area of expertise applies to everything, which it doesn't, or that his expertise proves that he's smart and therefore right about everything, even things he hasn't studied. One may have to be smart to understand nuclear physics, but it doesn't automatically teach you about, in my example, economics.
Or, in some fields, there is no absolute expertise. No one agrees about philosophy, or morality. I would not accept the Ayatollah's ideas about morality, and he would not accept mine. Our beliefs are just too different, and there's no objective way to prove that one is absolutely better. (Watch as I receive three tons of hate mail from the Ayatollah's friends and enemies for saying that.)
And now, if you'll excuse me, I'm back to trying not to flunk out of school.