According to Wikipedia:
Gender identity is the personal sense of one's own gender. Gender identity can correlate with a person's assigned sex or can differ from it. In most individuals, the various biological determinants of sex are congruent, and consistent with the individual's gender identity. Gender Expression typically reflects a person's gender identity, but this is not always the case. While a person may express behaviors, attitudes, and appearances consistent with a particular gender role such expression may not necessarily reflect their gender identity.
According to Twitter and Tumblr Addicts:
Gender identity is a form of social currency used to protect oneself against any and all criticism, no matter how based in scientific fact it is. It's also a shield used to protect oneself from truths that people refuse to listen to, and allows you to target straight white males for the condemnable crime of their birth. The more pronouns you tack on to your identity, the more protection you'll have. If you identify as a queer metrosexual non-binary Australian Bit Plane, then you've more protection than the average normie.