Gender identity is a complex topic, and there can be a great deal of confusion about the differences between sex, gender identity, and gender expression.
Sex refers to our biological differences, such as internal and external sex organs, chromosomes, and hormonal profiles. Gender identity is how we internally feel about ourselves, and gender expression is how we present ourselves with regard to dress, actions, affect, and how these fit with gender norms.
Gender identity conflicts arise when an individual’s biological sex differs from their gender identity and gender expression. These conflicts can develop at any age, but most often appear during childhood (5-8 years of age), adolescence, and early adulthood.
Transgender or trans* refers to an umbrella of identities, and trans individuals identify as experiencing a gender identity conflict. Trans is independent of sexual orientation, and trans individuals can identify as heterosexual, lesbian, gay, bisexual, or other.
Here a two books you might find interesting:
The Transgender Child: A Handbook for Families and Professionals
Helping Your Transgender Teen: A Guide for Parents
Dallas Counseling for Gender Identity