The "Straw Man" fallacy refers generally to the practice of refuting weaker arguments than your opponents actually offer. The terminology is based on a combat metaphor -- instead of grappling with your opponent's real arguments, you set up a straw man which is easier to knock down.
Of course, it is not a logical fallacy to disprove a week argument, the fallacy is believing or acting as though proving that one argument for a position is invalid proves that the conclusion is wrong.
People often refer to several different situations as "setting up a straw man."
- Presenting one of your opponent's lesser arguments then refuting it and then going on as if you've refuted her whole argument.
- Presenting a modified version of your opponents argument which is weaker than their real argument and then treating the refutation of the weaker argument as a refutation of her real argument.
- Presenting an poor defender of a position as the defender of that position and then defeating her arguments, and acting as than that were a refutation of all those who've argued for that position.
Some logic textbooks define the straw man fallacy only in terms of a misrepresented argument. However, it is now common to use the term in a much more general sense, and therefore we now identify all three situations as examples of the straw man fallacy.
/Talk