The straw man fallacy occurs when an arguer refers to an exaggerated, distorted or oversimplified version of the opposing position (or argument) which is more easily defeated than the actual opposing position (or argument).

Here is an example:

Rod: Being exposed to more and more people who have the flu raises your own risk of catching the flu.

Kit: Actually there are a variety of factors that raise your chance for catching the flu, including traveling, experiencing high stress, and not getting the flu shot.

Rod: So what you're saying is that exposure to sick people doesn't matter at all for risk of catching the flu. You don't know anything!

Kit: Wait, that's not what I'm saying.

In Rod's second remark he is putting up a straw man of Kit's actual opinion about the risk factors for catching the flu. Kit was not saying exposure "doesn't matter."

This fallacy is named "straw man" because a man made of straw is easier to "knock down" than a real, solid "man".