Not what your body is, but what your mind is. A person's gender identity is not always the same as their physical
sex, and in fact, can take on more than just the standard
male and
female roles, as it can be anywhere
in between the two, or as some may argue,
neither.
Currently, western society seems to assume that a person's gender identity must be identical to their physical sex, resulting in the suffering of those who cannot fit into the standard gender stereotypes. Attempts to act in manners considered inappropriate for the physical sex may often be rebuked, whether by family, friends, or strangers.
This is still a controversial idea. There are other points of view, such as the standard physical sex equals mental gender, or that a person's gender identity is completely shaped by social influences.