Gender Bending

Gender Bending is a broad category of events that can happen in fan fiction, involving playing with gender.

Opposite Gender AU
Writing an AU in which one or more characters are, and always have been, the opposite gender is an interesting way to explore the effects of gender on a storyline. Would a female Harry Potter still have the same relationship with family and friends? What if Elrond had had a son instead of Arwen?

Opposite-gender AUs are, like all AUs, required to be plausible and well-written.

Gender-Switch
In a gender-switch or gender-swap story, a magical, technological, or utterly random effect flips the genders of one or more characters, and they have to deal with experiencing life on the other side of the gender divide. Stories like this can be anywhere from stupid to funny or even thought-provoking. The webcomic "Misfile" is an example of a serious gender-switch story.

The 2009 Gender Bender Crisis was an incident involving gender-switching at PPC HQ.

Body-Switch
Rather than switching genders, a character switches bodies with someone of the opposite gender. Probably popularized by stories like "Freaky Friday", a body-switch story allows one character to literally walk in another character's shoes.

Cross-Dressing
Whether it's a disguise, an erotic activity, a form of entertainment, or just for fun, cross-dressing simply means wearing clothing typical to the other sex. Cross-dressers may attempt to pass as the other gender, or they may deliberately overplay the opposite-gender stereotype (as drag queens/kings do).

In fan fiction, cross-dressing is usually done for the purpose of disguise, and it can be funny... but woe to the fanbrat who uses feminine clothing to bash a male character, or vice versa.

Trans Characters
A transgender person has a mental gender that does not match the gender assigned to them at birth. They exist both in fan fiction and in real life, though in fan-fiction they may be a result of an unwanted gender-switch.

Badfic often includes plenty of trans!fail, usually involving the idea that the character is "really" their assigned gender, rather than their mental gender.

Some guidelines:
 * The mental gender, not the assigned gender, is the person's true gender. That means (if you think about it) that they are not actually "male trapped in a female body"--they are male; so their body is male even though it is atypical for a male (or vice versa).
 * Grammatically, pronouns that match the mental gender make the most sense, whether or not they have made any physical modifications.
 * Having someone find out the person they are dating is trans, recoiling in horror, and having the story playing it as funny? Not cool. Not cool at all.
 * The opposite of trans is "cis". No need to make up any more words than we have to.
 * Transwoman/transman work well if you have to specify, but there's usually no reason not to just refer to them as simply "man" or "woman".

Genderqueer Characters
This is another phenomenon that exists both in fan fiction and in real life. There are lots of variations, especially with magic and sci-fi technology involved, but genderqueer people are simply those who don't fit into the usual male/female categories. They may be in-between, switch periodically from one gender to the other, or simply not have a gender. They may be third-gendered. In fiction, entire species of shapeshifters may not even have a concept of gender.

"Genderqueer" refers mostly to people whose species has a relatively firmly-established gender binary that they don't fit into--while shapeshifters or non-gendered robots may be neither male nor female, and be effectively genderqueer, they are not usually called genderqueer.

Unfortunately, English has no good firmly-established gender-neutral pronouns. Those wishing to read more on the matter can start with Wikipedia's article on gender-neutral pronouns.

In the PPC, Laburnum/Hemlock is fond of switching genders, and may be considered either genderqueer or multiple-personality.