An informal fallacy is an error in reasoning that does not originate in improper logical form. Arguments committing informal fallacies may be formally valid, but still fallacious. An error that stems from a poor logical form is sometimes called formal fallacy or simply an invalid argument.
There are many different informal fallacies, but a few basic types. For instance, material fallacies is error in what the arguer is talking about, while Verbal fallacies is error in how the arguer is talking.
Fallacies of presumption fail to prove the conclusion by assuming the conclusion in the proof. Fallacies of weak inference fail to prove the conclusion with insufficient evidence. Fallacies of distraction fail to prove the conclusion with irrelevant evidence, like emotion. Fallacies of ambiguity fail to prove the conclusion due to vagueness in words, phrases, or grammar.
Some fallacies are committed intentionally (to manipulate or persuade by deception), others unintentionally due to carelessness or ignorance.