The following is copied from the American Psychological Association website

 

What does transgender mean?
Transgender is an umbrella term used to describe people whose gender identity (sense of themselves as male or female) or gender expression differs from that usually associated with their birth sex. Many transgender people live part-time or full-time as members of the other gender. Broadly speaking, anyone whose identity, appearance, or behavior falls outside of conventional gender norms can be described as transgender. However, not everyone whose appearance or behavior is gender-atypical will identify as a transgender person.
 
What is the difference between sex and gender?
Sex refers to biological status as male or female. It includes physical attributes such as sex chromosomes, gonads, sex hormones, internal reproductive structures, and external genitalia. Gender is a term that is often used to refer to ways that people act, interact, or feel about themselves, which are associated with boys/men and girls/women. While aspects of biological sex are the same across different cultures, aspects of gender may not be.
 
What are some categories or types of transgender people?
Transsexuals are transgender people who live or wish to live full time as members of the gender opposite to their birth sex. Biological females who wish to live and be recognized as men are called female-to-male (FTM) transsexuals or transsexual men. Biological males who wish to live and be recognized as women are called male-to-female (MTF) transsexuals or transsexual women. Transsexuals usually seek medical interventions, such as hormones and surgery, to make their bodies as congruent as possible with their preferred gender. The process of transitioning from one gender to the other is called sex reassignment or gender reassignment.
 
Cross-dressers or transvestites comprise the most numerous transgender group. Cross-dressers wear the clothing of the other sex. They vary in how completely they dress (from one article of clothing to fully cross-dressing) as well as in their motives for doing so. Some cross-dress to express cross-gender feelings or identities; others crossdress for fun, for emotional comfort, or for sexual arousal. The great majority of cross-dressers are biological males, most of whom are sexually attracted to women.
 
Drag queens and drag kings are, respectively, biological males and females who present part-time as members of the other sex primarily to perform or entertain. Their performances may include singing, lip-syncing, or dancing. Drag performers may or may not identify as transgender. Many drag queens and kings identify as gay, lesbian, or bisexual.

Other categories of transgender people include androgynous, bigendered, and gender queer people. Exact definitions of these terms vary from person to person, but often include a sense of blending or alternating genders. Some people who use these terms to describe themselves see traditional concepts of gender as restrictive.
Have transgender people always existed?
Transgender persons have been documented in many Western and non-Western cultures and societies from antiquity until the present day. However, the meaning of gender variance may vary from culture to culture.
Why are some people transgender?
There is no one generally accepted explanation for why some people are transgender. The diversity of transgender expression argues against any simple or unitary explanation. Many experts believe that biological factors such as genetic influences and prenatal hormone levels, early experiences in a person’s family of origin, and other social influences can all contribute to the development of transgender behaviors and identities.