Gender identity is one's internal sense of being male, female, non-binary, or another gender, while expression is how one presents that identity to the world.
This is a personal process that can involve social changes (like names and pronouns), medical changes (hormones or surgery), or legal changes (updating documents). History of Transgender Activism
"Transgender" is an umbrella term for individuals whose gender identity or expression differs from the sex they were assigned at birth.