A straw man is a misrepresentation or distortion of an argument, view, or statement created so that its opponent has an easier target to destroy. Straw men rarely convince the party promoting or defending the original position, but those who agree with or are inclined to agree with the user of the straw man probably won't know the difference between the straw man and the real thing, and will perceive the demolition of the straw man as a defeat of the real thing. Those prone to polar thinking may also unintentionally create straw men of their opponents' positions.
An an example of straw manning, let's say that Corbin states that he doesn't like the character of Ruby Lann from the hit sci-fi TV show StarRunners, citing that Ruby is absurdly overtalented and overpowered and that every time she appears in the story, the plot derails and everything becomes about how beautiful, smart, tragic, and perfect Ruby is.
Then, Ruby Lann fan Jessica gets wind of Corbin's complaints about the character. Angry that someone out there doesn't like her favorite character, she goes to her friends to complain - "Can you believe that Corbin guy? He hates Ruby Lann because she's a female character who dared to be beautiful and talented! What a hypocritical sexist pig!" (Never mind that Corbin has never complained about other characters in the show who happen to be female, beautiful, and talented.)
Or, Ryan submits his character profile to a roleplaying site. The admins review it, and decide that Ryan's dark elf/vampire/dragon/demon hybrid with a soul-stealing sword and the ability to control the four elements is both severely overpowered and out of place in their Star Trek roleplay. So, Ryan throws a fit - "I just wanted to have fun - isn't that what roleplaying is supposed to be about? What's your peoples' problems with people being creative and having fun, anyway?" (The admins don't have anything against fun or creativity - they want reasonably balanced characters that fit into the Star Trek universe. If they wanted to play Uberpowered Fantasy Mishmash, that's what they'd be playing already.)
Or, many opponents of evolutionary biology use straw man arguments against it, such as telling people that "evolution says" lightning struck a mud puddle which caused the first cells to form, or that if evolution were true then we should see dogs giving birth to cats and monkeys evolving into humans. However, the theory of evolution says nothing so absurd, and no respectable biologist would ever say it does. (For the curious, there are links to information on how evolution actually does work on the bottom of this page.)