Gender is a social construct, and can differ from one's biological sex (transgender), and has values typically associated with it such as masculinity, femininity, or androgyny. Some people don't express themselves with the attributes generally applied to their gender; for example a trans woman can express themselves in a masculine manner while still being a woman.
Gender versus Sex
Sex is a biological value, generally determined by genitalia or chromosomes, for example a person with a penis is generally assigned male at birth. Biological attributes influenced by sex can differ, which is known as intersex.
How many genders?
Gender is a infinitely large spectrum, with many positions and identities. A simple way of looking at it is male and female, and everything in between being a term to describe your relationship with gender, for example demiboys being a term to describe being partially masculine and androgynous however, this model is still flawed as it does not encompass the full range of potential gender experiences, such as abinary and atrinary genders.