Informal logical fallacies are instances of fallacious reasoning which are not due to the logical form of an argument, but rather are specific to the substantive content of one or more of the propositions in the argument. As such, whether or not an argument commits a given informal fallacy can only be judged by examining the particular content of the premises of the argument. It cannot be judged by examining the logical form alone, as is possible for formal logical fallacies.
Informal fallacies are much more common in real world arguments than formal fallacies, in part because often they are misapplications of otherwise valid forms of reasoning. For example, appeals to authority or slippery slope arguments are not always fallacious, depending on the context and particulars of the argument. As such, judging whether or not informal logical fallacies have been committed typically requires practise and skill. Some people are too ready to declare that any argument whose conclusion they dislike commits one or more informal fallacy, and in so doing often exhibit a misunderstanding of what the argument in question is actually asserting, or how the conclusion is supposed to follow from the premises. Before declaring an argument fallacious, therefore, we should be sure that we have carefully examined the premises and the structure of the argument so that we can be sure we are not misconstruing the intent of the argument. Despite this caution, informal logical fallacies are exceptionally common in everyday reasoning, and a familiarity with some of the major forms they can take can significantly improve one’s ability to reason correctly, identify fallacious arguments, and construct valid arguments of one’s own.
Informal Fallacy: a useful introduction from the Fallacy Files
Fallacies: a brief introduction and list of fallacies from the Nizkor Project
Fallacies: an excellent discussion of some of the key issues surrounding the definition and use of informal fallacies