Everyone has a gender identity. For some people, gender identity comes with the gender assigned at birth, while for others it doesn't. Gender identities differ from the sex assigned at birth, including those beyond the binary.
Gender identity refers to a person's understanding and experience of their gender.
Gender identities are broad and need not be limited to a single agreed term. There is no authority that determines the boundaries of a gender other than the individual concerned.
Gender expression refers to the ways a person chooses to present their gender to the world around them. This includes clothing, mannerisms, pronouns, nouns...However, it's important to state that while things like names, clothing, and others are part of a person's gender expression, they are not necessarily a gender attached to them. Additionally, a person's gender identity can sometimes inform about a person's gender expression, a person's perceived gender expression does not dictate their gender identity.