A straw man argument is an informal fallacy of relevance where an arguer distorts or misrepresents his or her opponent's position in order to more easily refute it. The obvious fallacy with this sort of argument is that the weaker, misrepresented position which is being refuted (the straw man) is not the actual claim being made.
Though the etymology of the term is unclear, a commonly stated origin is that it comes from the past use of straw man dummies by the military to train soldiers. Though the dummy represents the enemy it is of course a far easier opponent to defeat than a real person.
The straw man argument has the following form:
Person A has position X.
Person B presents position Y (which is a distortion or misrepresentation of position X).
Person B then attacks position Y (the so called straw man).
Therefore position X is false.
Bob says "It would be a good idea to ban advertising beer and wine on radio and television. These ads encourage teenagers to drink, often with disastrous consequences."
John replies "You cannot get people to give up drinking: they've been doing it for thousands of years."
Bob did not propose that people give up drinking yet John misrepresents it as such providing him with an easier argument to refute.
Methods of Argumentation
The Nizkor Project: Fallacy: Straw Man