Transgender (trans, trans*) is an umbrella term for people whose gender identity differs from the sex assigned to them at birth. The key to understanding what transgender means is understanding the difference between sex and gender.
Sex refers to the physical characteristics that are associated with being male or female, including primary sex characteristics (such as genitals) and secondary sex characteristics (such as breasts).
Gender refers to the social presentation of masculinity and femininity. Many cultures have strict rules about how to perform masculinity and femininity.
Rigid masculine and feminine gender roles are referred to as the gender binary. However, ideas about gender are not static—they change across time and place, within one society and between different cultures.
Transgender individuals do not identify with the sex that they were assigned at birth, and present their gender in a way that reflects their true selves. Some transgender persons choose to have surgeries so that their physical characteristics reflect their gender identity, and some do not. MtF describes someone who has transitioned from male to female. FtM describes someone who has transitioned from female to male.
Gender identity does not relate to sexual orientation. Transgender or Gender-Variant people may identify as straight, gay, lesbian, bisexual, pansexual or another sexual orientation.