A Straw Man argument is one that attempts to refute a position an opponent hasn't actually taken. Typically the straw man will be an extreme position or, as Bruce Thompson notes, an alleged hidden agenda.
Sometimes the straw man is simply the result of a misinterpreted position. But no matter how well the straw man is defeated, the true position remains unassailed. The Straw Man can be seen therefor as shifting the burden of proof.
The argument misrepresents a position that it seeks to refute. By refuting the position as misrepresented, the argument creates the impression that it has refuted the position that is actually held by opponents.Fallacy Files:
As the "straw man" metaphor suggests, the counterfeit position attacked in a Straw Man argument is typically weaker than the opponent's actual position, just as a straw man is easier to defeat than a flesh-and-blood one. Of course, this is no accident, but is part of what makes the fallacy tempting to commit, especially to a desperate debater who is losing an argument.