Gender identity is an internal sense of being a gender, regardless of physical characteristics (sex), intentional appearance and behavior (gender expression), or sexual orientation. Most people identify as the gender that they were assigned at birth, which is called being cisgender. Transgender people don't identify as the genders they were assigned at birth.
See main article: binary genders
Western society uses the gender binary system of categorizing all people into only one of the binary genders: female (woman) and male (man). The gender binary system is coercive and limiting, but the binary genders themselves are valid identities.
See main article: nonbinary gender
Nonbinary gender is any gender that exists outside of the gender binary. That is, any gender not described by just "male" or "female". Even in societies that recognize only two genders, there are still people who find that they don't fit into those two. Despite the lack of recognition of nonbinary genders in Western binarist society, nonbinary genders are still valid identities.
Gender identity can remain the same or change over the course of an individual's life.
Genderfluid people experience frequent changes in their gender identity, as often as monthly, weekly, or even several times a day. Genderfluid is a broad category of gender identities that share the characteristic of frequent change. This category can include multigender identities, such as bigender, depending on how an individual defines themself under those terms.
However, other people have the same gender identity throughout their entire life, with little to no change. It depends on the individual.