A 'straw man' is an undefensible position. Basically, you throw out a straw man, watch people agree that it's a bad thing, and then say their entire argument is flawed.
Perhaps an example is in order: Say we're arguing about the merits of nuclear power. I say "Do you want another Chernobyl?" Obviously, you don't, and you say so. Then I say "Well then, all nuclear power is bad!"
I have just committed a straw man logical fallacy.
A fairly complete list of fallacies:
http://www.infidels.org/news/atheism....html#strawman