A logical fallacy is an error in reasoning common enough to warrant a fancy name. Knowing how to spot and identify fallacies is a priceless skill. It can save you time, money, and personal dignity. There are two major categories of logical fallacies, which in turn break down into a wide range of types of fallacies, each with their own unique ways of trying to trick you into agreement.
A Formal Fallacy is a breakdown in how you say something. The ideas are somehow sequenced incorrectly. Their form is wrong, rendering the argument as noise and nonsense.
An Informal Fallacy denotes an error in what you are saying, that is, the content of your argument. The ideas might be arranged correctly, but something you said isn’t quite right. The content is wrong or off-kilter.
Example of Ad Hominem Argument
A politician arguing that his opponent cannot possibly be a good choice for women because he has a religious conviction that causes him to be pro-life.
"Correlation is not causation" means that just because two things correlate does not necessarily mean that one causes the other. Correlations between two things can be caused by a third factor that affects both of them.
While appeals to authority (Executive Directors, Board Members) are by no means always fallacious, they can quickly become dangerous when you rely too heavily on the opinion of a single person -- especially if that person is attempting to validate something outside of their expertise.