A logical fallacy is usually what has happened when someone is wrong about something. It’s a flaw in reasoning. They’re like tricks or illusions of thought, and they’re often very sneakily used by politicians and the media to fool people.
And this is a site that beautifully presents and explains all these little cognitive traps. The explanations are a bit on the short side, though. More in-depth ones can be found at youarenotsosmart.com. (That’s just about the best domain ever, don’t you think?)