A logical fallacy is an argument that doesn't hold together. All of its assumptions might be true, but the conclusion doesn't actually follow from them.

"Fallacy" is used to mean a few different things, though.

Formal fallacies happen when you try to prove something with a logical argument, but the structure of your argument is broken. For instance, "All weasels are furry; Spot is furry; therefore Spot is a weasel." Any argument of this "shape" will have the same problem — regardless of whether it's about weasels, politics, or Java programming.

Informal fallacies happen when you try to convince people of your conclusion through arguments that are irrelevant. A lot of informal fallacies try to argue that a statement is true because of something else — like its popularity, or the purported opinion of a smart person; or that its opponents are villains.