In logic, a fallacy is one of many various types of erroneous reasoning that render arguments logically unsound. In more general terms, one can view a fallacy as a misleading or unsound argument.
People can, and do, use fallacies to purposely trick others into agreeing with their side of a given argument. Most often, however, mistakes in logic are not even noticed by those that have them, as the emotional connection to a given viewpoint weakens their ability to view said viewpoint from a neutral perspective.