Transvestite (TV): A term for those who feel compelled to wear the clothes of the opposite gender, who often feel 50% or more female, but would never change sex. Used indiscriminately, but often used faintly disapprovingly (unless abbreviated to "Tranny"). Hence (usually incorrectly) associated with homosexuality. The word has clinical or medical overtones, which quite wrongly imply a "perversion" to be "cured". Crossdressing is in fact a harmless alternative lifestyle, which for most is a part-time hobby.
Crossdresser (CD): As TV above but less judgemental, implying acceptance or tolerance. No sexual implications. A good word, especially for heterosexuals. (Not suitable for transsexuals - see below).
MTF = Male to Female (the majority), FTM = Female to Male (fewer, because it's relatively easy for a woman to get away with looking masculine). RTV = Regular Transvestite, OTV = Occasional, FTV = Full Time, RG = Real Girl (please not GG which = Horse!).
Gender Transient: Someone who swaps between male and female clothes and behaviour at will.
Transsexual (TS): Someone born with a female brain in a male body, or vice-versa. "Gender dysphoria" can only be cured by surgery and hormones to align the body with the brain ("gender reassignment"), whereupon the person is no longer TS. She normally lives in "stealth mode", i.e. full time as a woman for some time even before surgery, to get used to the life she was meant for.
PTS = Post- (not Pre-) Operative Transsexual. To avoid possible offence, these women should always be referred to as female. "PTS" should be used only when essential to specify that current gender assignment differs from birth gender.
Drag: Dressed As a Girl. Drab: Dressed As a Boy.
'Drag' implies Drag Queen, Gender-bender: Female impersonator, usually on stage. Extrovert, exhibitionist, parodies women only to entertain. Usually gay, does not usually look real; particularly in that despite the glamorous clothes, mannerisms are exaggerated, or still masculine.
Bisexual: Fancying both men and women. Correct meaning = "of both sexes", but has come instead to mean "Swings both ways", or "AC/DC".
Note that most gay men (contrary to popular belief!) hate the idea of dressing as women. Even those who do, normally fancy butch men, not TVs or TSs. Gay men often criticise crossdressers to draw attention away from their own secret leanings. Gay women sometimes oppose crossdressing, as they only like real women. People of all kinds who are comfortable with their own sexuality tend to be the most supportive of gender issues.
Sissy: A male who fantasises about enforced femininity. This is really about submissive role play, such as being in a domestic maid to a female dominatrix; or for a tiny minority, being innocently dressed as an adult girl child.
She-Male, Ladyboy: Someone with male genitalia who has taken hormones to give themselves a more feminine body. Common in Thailand and Brazil. Terms (rather unfortunately) associated with prostitution in underdeveloped countries, usually due to political and police oppression.
Eunuch: The castrated but widely accepted "third sex", who have been around for centuries around the world and are still numerous in some countries such as India.
Intersex, Hermaphrodite: Rare. Someone with ambiguous sex organs, or with those of both sexes. Usually corrected by surgery in infancy, but he or she may need to remain on hormones for life.
Transgendered: A useful and polite term to include all of the above.