Bad arguments do not announce themselves. They arrive sounding sensible, often from people who are confident and articulate, and they exploit habits of thought that usually serve us perfectly well. A logical fallacy is the name for the specific way such an argument goes wrong — and having names for them turns a vague feeling that something is off into a precise diagnosis.
Philosophers divide them into two families. Formal fallacies are structural: the argument's shape is invalid, so the conclusion does not follow no matter how true the premises are. Informal fallacies are failures of content — irrelevant appeals, misrepresented positions, loaded language, missing evidence. Almost every fallacy you meet in an actual disagreement is informal, because real arguments rarely fail on pure logic; they fail on relevance and honesty.
The essential listThe fallacies worth knowing by name.
The cautionNaming a fallacy is not winning.
There is a failure mode that arrives fast once you learn this vocabulary, and it deserves its own warning. Identifying a fallacy shows that this particular argument fails. It does not show that the conclusion is false. Someone can defend a true claim with terrible reasoning, and dismissing the claim because the reasoning was bad is itself an error — the fallacy fallacy. The point of learning these patterns is to reason better, not to acquire a faster way to dismiss people.
The second caution concerns charity. Many apparent fallacies dissolve when you state the other person's argument as strongly as you can before responding — the discipline this whole site is built on. A slippery slope with a genuine causal mechanism is not a fallacy; it is an argument. An appeal to a relevant expert is not a fallacy; it is ordinary good practice. What makes each version fallacious is a specific missing element, and knowing which element is missing is the actual skill.