Fallacies

Reductio ad Absurdum is often confused with the Appeal to Ridicule and/or with Straw Man.
Straw man – a fallacious argument based on misrepresentation of an opponent's position (e.g. if evolution was true, there would be no monkeys)
Appeal to ridicule – a fallacious argument made by presenting the opponent's argument in a way that makes it appear ridiculous (e.g.  if evolution was true, why are there still monkeys?!).

But *Reductio ad Absurdum* without misstating the argument:
1 - Start by assuming P is true.
2 - Deduce a contradiction with the assumed premise or with a known feature of reality.
(There is no smallest positive rational number, because if there were, then it could be divided by two to get a smaller one.)

Or more formally it takes the following structure:
1 - Start by assuming P is true.
2 - From this assumption, deduce that Q is true.
3 - Also deduce that Q is false.
4 - Thus, P implies both Q and not Q (a contradiction, which is necessarily false).
5 - Therefore, P itself must be false.

http://en.wikipedia.org/wiki/List_of_fallacies
http://en.wikipedia.org/wiki/Reductio_ad_absurdum
http://en.wikipedia.org/wiki/Straw_man
http://en.wikipedia.org/wiki/Appeal_to_ridicule