A fallacy is a flaw in reasoning that undermines the logic of an argument. It can occur in various forms, such as making assumptions without evidence or misrepresenting an opponent's position. Fallacies can be intentional or unintentional and often distract from the actual issue being discussed. Common types of fallacies include the straw man fallacy, where someone misrepresents an argument to make it easier to attack, and the ad hominem fallacy, which targets a person's character instead of addressing the argument itself. Recognizing fallacies is essential for critical thinking and effective communication.