Our physical bodies define our sex but our gender identity describes the sense of knowing that we are men or women, and gender role describes how we behave in society. None of these things are to do with our sexual orientation; whether a person is sexually attracted to men, or women, or both or neither.
Transgender people find that the body they have on the outside doesn’t fit how they feel inside and this can cause extreme discomfort and unhappiness.
There are many different ways for people to cope with these feelings. Some people find that dressing in clothes designed for the other sex and living full-time in that gender role is enough and never need to change their body using medical treatment. Some people do not identify comfortably as either men or women, but feel that they have characteristics of both and live somewhere in between.