The above writeups are technically correct but, I feel, miss out on a possibility: the unintentional genderfuck.
Here's the skinny:
I'm genderqueer and proud of it; if I weren't proud of it then there's a danger that I would become very, very sad (TM). Like almost everyone in the Western world, I have a favourite set of clothes, which I will wear when I want to look nice; it's nothing fancy: tight pants, a red top, some makeup and bothering to style my hair so it looks aggressive rather than just fluffy.
I end up androgynous, with a flat chest but big hips. My face doesn't give too much of a clue; I've practiced male and female expressions in the mirror and I'll use them both as I please, so if you start looking at me at 10:00PM, you might see a different person than if you'd started looking at 9:50PM (first impressions tend to stick, though). I'm prettier if I conform to the gender corresponding to my sex, but I've sparked drunken arguments between strangers about what I am. Whatever.
It's more fun than I expected; one feels like one is performing threefold service: To the people you fuck, by challenging their expectations. To the trans community by fucking the squares. To yourself by reminding you that not all strangers are bad people; a look of confusion isn't too bad, and I've been lucky or sensible enough to not put myself where I don't feel safe.
But it's not intentional; I was just chillin' at home last week, watching some TV in some very old sportswear (from waay back when I was a good little larvum), and I answer the doorbell to a salesman (sales-boy, really) touting a sports-bag of whatever he was selling around on his back. His opening line to me?
"Good evening, sir or madam."
He gets full marks, for cojones if nothing else. If I could stop, I don't know if I would. My point is that the writeups above assume that a genderfuck needs to be some kind of statement - some kind of action which one has decided to do. It really doesn't have to be - when I get dressed, I try to conform as much as I can - sticking out isn't a good definition for a person to have, and I still fail.
Of course, there's a worse kind of unintentional genderfuck - when you're trying to pass as the opposite sex and fail. I guess this is similar to above.