A Straw Man is a logical fallacy. If you're not familiar with logical fallacies, there's an excellent summary over at the Skeptic's Guide to the Universe, which is worth reading. In summary, a logical fallacy is an incorrect argument, normally due to an error in logic or rhetoric, and either purposeful or accidental. There's a massive list of formal fallacies, because certain errors just pop over and over again in argument.
Straw man tends to evoke the image of soldiers bayonetting straw dummies, and it's used for a reason. In the straw man logical fallacy, instead of arguing against your opponent's actual position, you make up an entirely different, misrepresentative position, one that's normally a lot easier to attack, and then argue against that instead. Politicians do it all the time by putting words in their opponent's mouth or purposefully misinterpreting something they've said.
I've attempted to come up with some real world examples, but feel free to offer your own in comments, or correct me if I've made a mistake.
- Just about any politician ever who has claimed that anyone opposed to Law X just wants to maintain the status quo. George W. Bush did that when he argued for No Child Left Behind, Obama did it for the Affordable Care Act. While some opposition comes from wanting to keep things as they are no doubt, there were plenty of people opposed to both because they thought they didn't go far enough (hey, wouldn't it be awesome to have single payer?) or that it went about things the wrong way (way to require mathematically impossible rates of success in testing!) and cogent arguments to that effect.
- Arguing against feminism because we're all just a bunch of man hating bra-burners who want to put a matriarchy in place is one that happens all the time. So often, in fact, the Straw Feminist is a trope.
- Just about anything to do with the Fox News fictional "War on Christmas." (Example here.) I would argue that the entire concept of the "War on Christmas" is a straw man, since it's a characterization of people wanting to destroy the holiday when basically the evil opposing forces (anyone who says "happy holidays" and atheists for the most part) most commonly want to make the holiday season inclusive for all faiths and/or feel that the government shouldn't be promoting a particular flavor of religion.
- A popular one currently is to characterize any discussion of regulations on guns as an attempt to ban all guns forever. I've gotten hit with that on both my post Sandy Hook massacre blog posts; note in neither do I say anything about banning guns entirely.
Logical fallacies are a powerful, useful tool. Two things you need to keep in mind however:
- Just because an argument contains a logical fallacy does not mean that the conclusion will necessarily be false. Sometimes someone with shitty logic still gets to a correct conclusion. Also, sometimes people will purposefully commit these fallacies as a rhetorical device, so be cautious of that as well.
- Just because you know the logical fallacies does not mean you are immune to committing them yourself. I know I've fallen victim to their siren call in the past. So don't let it get to your ego, okay?