Sex and gender are often used interchangeably, but the terms have different meanings. Biological or genetic sex is determined at birth, typically by the physical appearance of the genitalia.
Gender refers to an individual's outward appearance and behavior and, more importantly, self-identification as male or female. For most people, sex and gender correspond.
Transgender is the umbrella term coined in the 1970s to refer to people who do not identify with the sex designated at birth. The term encompasses transsexuals, cross-dressers and (in some usages) intersexed persons.
A transsexual — an older term — is someone who changes his or her original sex through medical procedures. A transwoman, or MTF, changes from male to female; a transman, or FTM, from female to male.
Hormone-replacement therapy is the use of testosterone or other male hormone for FTMs or estrogen for MTFs to produce some of the secondary sex characteristics of the gender the person is transitioning into, such as facial hair for FTMs or breast enlargement for MTFs. Some secondary sex characteristics — notably, voice modulation for MTFs — are changed only through instruction and practice.
Sex-reassignment surgery entails a mastectomy (removal of the breasts) for FTMs (“top surgery”) and may entail genital reconstruction for either MTFs or FTMs (“bottom surgery”). For MTFs, complete reassignment surgery includes removal of the testes (castration) and penis (penectomy) and creation of a vagina and labia (vaginoplasty). For FTMs, the surgery includes creation of a penis by using the clitoris and its surrounding tissue (metoidioplasty) or tissue from other parts of the body (phalloplasty). Many, perhaps most, transsexuals do not undergo genital surgery; in particular, the results of a phalloplasty are not completely satisfactory, although surgical techniques are said to be improving.
Sexual orientation refers to an individual's physical or affectional attraction to individuals of the same sex, opposite sex or both. Transgender people have varying sexual orientations: Some are attracted to persons of the opposite gender (“straight”), some to persons of the same gender (“gay” or “lesbian”), and some to both (“bisexual”).
A cross-dresser likes to wear clothes associated with the opposite gender but does not want to change his or her physical sex. (The older term “transvestite” is now widely viewed as pejorative.) Most male cross-dressers prefer female sex partners.
Intersex refers to someone born with ambiguous genitalia, an abnormally small penis or abnormally large clitoris or other atypical reproductive or sexual anatomy. Experts estimate that 1 percent of the population is born with this condition. (The older term, “hermaphrodite,” is now viewed as pejorative.) Chromosome testing may be needed to designate their sex (XX chromosome for females, XY for males). Genital surgery may be used to attempt to create genitalia corresponding to the designated sex. The common practice of performing this surgery with the parents' consent soon after birth or in infancy is now viewed with disfavor by some intersexed persons.