Definition and Common identities
Gender Identity is how a person identifies themselves. A person’s gender identity is based on what label and type they feel most comfortable aligning with. It can change throughout someone’s life, depending on how they feel during that time. Much of dominant society across cultures follows the construction of the gender binary, prescribing that everyone must either be a man or a woman. However, more genders exist outside of male and female. Recent decades have brought new terminology and means of using language to make space for a range of genders. And more importantly, the rise of LGBTQ+ education and rights have brought transgender issues to the attention of the public, as well as the many genders that a person can identify as.

 Cisgender: the label for people who identify as the gender they were assigned at birth (ex: male and female)

'Transgender' is both an identity label and an umbrella term. A person can identify as a trans woman or trans man, but there are more genders that fall underneath the transgender category. Nonbinary genders (or people who do not identify as male or female) may fall underneath transgender, depending on how the person self-identifies. 'Nonbinary' may be used as a synonym for agender, or someone doesn't identify with a gender, but also may refer to other genders. 

Some gender identities include, but aren't limited to:

 Agender
Bigender
Cisgender
Genderfluid
Genderqueer
 Nonbinary
Transgender
Two-Spirit
 and more.

Gender expression
Gender expression (also called, "gender presentation") means how a person dresses, looks, and acts, in ways that might affect how other people view their gender. Someone who wears men's clothes and acts in a masculine way has a male gender expression.  Someone who wears women's clothes and acts in a feminine way has a female gender expression. This is different from gender identity because people can choose to look or behave one way even if that is not how they feel inside. Sometimes people call this gender presentation or just presentation.

Gender Identity in Anthropology 
Gender is a key concept in the field of anthropology. Gender and sex are defined differently in this field. Gender in regards to an individual's identity, and sex being biology determined. Gender identity, while can be tied to sex and biology in some, is not necessarily determined by these factors. Assumptions around gender identity based in biological factors can be false and while sometimes accepted in the mainstream, can be in fact dangerous. The roles and expectations of gender have strong cultural influences as well. Gender across cultures has been studied extensively, regarding the different identities accepted in the mainstream, roles of the genders, and historical trends of them. Gender Identity and expression has extremely common historical roots, and more gender identities outside of the traditional binary are becoming accepted in the mainstream. Acceptance of gender identities that do not exist within a harsh binary is not exclusively a new concept, just one that is becoming more common in current western society. There are various historical examples of gender identities that were not only recognized, but celebrated, outside the traditional cisgender male and female.

Transgender and transsexual
'Transgender' is a term that means that the gender assigned at birth (typically 'AMAB' or 'assigned male at birth' and 'AFAB' or 'assigned female at birth') does not match with the gender a person identifies as. The experiences that transgender people have are vast and may differ from one another, although there are sometimes shared experiences. For instance, a transgender person may feel gender dysphoria (but not everyone will be dysphoric). A transgender person may also feel it necessary to medically transition, or take hormones, or a combination of the two. 'Transsexual' is a term that some transgender people may use after receiving sex-reassignment surgeries and medical transition, but depending on who one is talking to, this term may be considered outdated and the person may instead opt to simply continue using the term 'transgender' as their identification.

Some people use the word 'transgender' to mean somewhat different things, as the word has expanded and changed since it was first invented in 1965.

Factors and terminology 
Some factors involved in transgender identification are:
Assigned gender (or sex assignment, or just sex) is whether a person is male or female at birth.
Gender identity (or simply gender) is the label a person uses to describe how they identify to the world around them: male, female, nonbinary, and other identities that fall around them. 
Gender expression (or gender presentation) is how a person dresses, acts, and behaves. It's important to remember that gender expression is not the same as gender identity. A cisgender woman can wear traditionally-male clothes and not be a man, a cisgender man can act in a way that's considered "feminine" and not be a woman, etc.

All three of these factors contribute to how a person labels their self, and make gender a confusing construct to navigate. People can have gender presentations that don't match their gender identity, or a gender identity that doesn't match the gender they were assigned at birth, etc. A person whose gender identity matches the gender they were assigned at birth is called 'cisgender'. As time has passed, the growing voice of LGBTQ+ issues and rise of educational resources on the topics of gender and sexuality has allowed for many to learn about transgender issues, and help others with their own gender navigation.

Types of identity mismatch

It is possible to have every different combination of sex, gender identity, and gender expression. For example:

 A person may be female at birth, but have a male gender identity, may call themself a transgender man or 'trans man'. Trans men may typically dress in a masculine manner, wear a chest binder, take testosterone and have sex reassignment surgery if they wish to do so.
 Someone who identifies as male (whether he was that at birth or has taken on that gender identity) may dress up and perform in traditionally-women's clothes (in drag). While in drag, this person performs as a woman and may call themselves such.. It also works vice-versa, where women perform on stage as men. This is called being a drag king.

There are many people whose gender identity differs from the one they are at birth, but the person hides it due to fear of rejection, laws that do not protect transgender people, being abandoned or cut off by family and friends, and even fear of getting assaulted.

For instance, a woman going on a date may have to grapple with the decision of when and or if she should tell her date that she is transgender. There is the chance that her date will not care that she is transgender and they can carry on, and she will be happy knowing that she can safely be herself around this person. However, there's also the chance that her date will react angrily and misgender her after finding out she is transgender, and maybe even assault her. A person who identifies as nonbinary may keep their identity a secret to their family out of fear of being kicked out and left homeless, or sent to conversion therapy.

Many people go through this fear, so it is very normal to feel scared.

Another issue that has come up recently is one that feminists, who believe one can dress however they want, argue over whether someone claiming to be a woman and deciding to wear a dress
should be treated as a woman based on this self-identification. This is a matter of dispute within the feminist movement. You can wear whatever you want and still be a woman. A transwoman AMAB who wears a dress makes it so that the feminist movement can not move past this idea that women must wear a dress. Women who see themselves as feminists have strong views on both sides of the issue of whether or not trans women are women,to the point that they deny those who hold the opposite view are real feminists.

Women who see feminism as based on fighting against the oppression of biological women based on biological sex often call themselves gender critical or gender abolitionists (the latter idea based on the expectation of particular dress or behavior being itself oppression). Those who disagree often label these women Trans-Exclusionary Radical Feminists, or TERFs.

Medical diagnosis
When a person's gender identity and body do not match, they may go to see a doctor. The doctor may help them change their body if that is what they want. A psychiatrist may diagnose this person with Gender identity disorder (gender dysphoria), but medical diagnosis itself is a controversial subject because being transgender has one been considered to be a mental illness, the stigma is still there. Principle 18 of the Yogyakarta Principles, a document about international law on human rights states that "any classifications to the contrary, a person's gender identity is not in and of itself, medical condition". And "Activist's Guide" to them says that "gender identity" or "gender identity disorder" exists still in categories of mental illness, contrary to the "sexual orientation" removed from such categories.

But despite the strides taken to remove the stigma of mental illness in being transgender, gatekeeping by doctors and therapists make most transgender people struggle to achieve their ideal self. In most cases, doctors require therapists to 'diagnose' whether their patient is 'transgender enough' to take hormones or have surgery. And even if you do that diagnosis, a surgeon can refuse to perform surgery on you.

References

Other websites
BORN FREE AND EQUAL - Sexual orientation and gender identity in international human rights law, 2012

Gender