What are logical fallacies?

http://www.logicalfallacies.info/

“Once it has been decided what is to count as a logical fallacy, the question remains as to how the various fallacies are to be categorised. The most common classification of fallacies groups fallacies of relevance, of ambiguity, and of presumption.

Arguments that commit fallacies of relevance rely on premises that aren’t relevant to the truth of the conclusion. The various irrelevant appeals are all fallacies of relevance, as are ad hominems.

Arguments that commit fallacies of ambiguity, such as equivocation or the straw man fallacy, manipulate language in misleading ways.

Arguments that commit fallacies of presumption contain false premises, and so fail to establish their conclusion. For example, arguments based on a false dilemma or circular arguments both commit fallacies of presumption.

These categories have to be treated quite loosely. Some fallacies are difficult to place in any category; others belong in two or three. The ‘No True Scotsman’ fallacy, for example, could be classified either as a fallacy of ambiguity (an attempt to switch definitions of “Scotsman”) or as a fallacy of presumption (it begs the question, reinterpreting the evidence to fit its conclusion rather than forming its conclusion on the basis of the evidence).”