This is a discussion board post and I expect all future posts to meet the minimum criteria. Further crappy/low content posts will be deleted and infracted.
Genders appear to be a cultural construct, not a scientific one. From what I gather, genders become relevant only when discussing "Types" or "Roles" where certain genders are associated with liking and doing certain things. In that respect, a gender and a sex are far from interchangeable terminology, and can be non-binary.
Sex on the other hand is almost exclusively influenced by genetics. A person's sex can be male, female, or in very rare cases intersex. It's essentially binary.
Some people feel uncomfortable when their sex and gender don't match. If you're a a male sex, and you love dresses, make up, cooking, shades of pink and yellow, maybe you don't feel very connected with the identity of a "boy". In this case, it makes sense to change your gender identification, and feel more at peace with your situation as a woman.
Some people aren't sure where they feel comfortable yet, and experiment. I suppose that term is referred to as gender fluidity, where they don't concretely feel masculine or feminine for too long and haven't settled in on a particular archetype.
I'm pretty much against genders. It really only serves to divide us. For practical purposes, people of similar physiology may want to be grouped together for fairness in some outlets such as professional sporting competitions but other than that, nobody should feel uncomfortable for being themselves.