Transgender is a broad concept often used to refer to people whose identity does not correspond to the sex they were assigned at birth. It should not be confused with transsexual. Transgender people do not necessarily undergo hormone therapy or surgeries to modify their physical characteristics.
The concept of transgender emerged in the 1970s as a critique of psyhco-medical authorities and their pathologizing definitions. In the 1990s and in 2000, transgender included all identities and experiences that today fall under the word trans. In order to treat a transgender person with respect, they should be referred to using their gender identity and not the sex they were assigned at birth.
Some people identify themselves as transgender. Many of them face transphobia on a daily basis, as they are socially perceived and identified based on the sex they were assigned at birth. Some groups of transgender women are now revindicating their recognition as transgender people. This is a neologism that includes the feminine nature of their identity.
References:
Prosex, ADC, FELGTB, Informe Trans, Guía para el tratamiento periodístico responsable de identidades de géneros, orientación sexual e intersualidad
Informe Trans, FELGTB, Transnational center for transgender equality.
Photo credit: Nadia Snopek