Sex is the biological male or female, as determined by the presence of xx or xy chromosome pairs. There are actual chromosomal disorders, such as Turners syndrome, where there are missing or extra chromosomes, and this can create an androgynous appearance. Generally, though, the word 'sex' refers to a binary male or female. Gender refers to socially constructed ideas about what sorts of behaviors represent a certain gender. In our society, we have two genders, masculine and feminine, and have expectations for each gender. For example, sports are supposedly masculine, while dolls are feminine, even though there is nothing biologically preventing a boy from liking dolls, and vice versa. These are just ideas created by society over many generations, and change along with other cultural and social changes. Gender IS a social construct, sex is biological. Humans have two sexes, but some cultures do have more than two genders. Some plains Indians back in the day had a third gender, which consisted of biological males who chose to take on the roles of females, and who could even be married to other men, yet they would not be considered to be 'men pretending to be women', there is just a third category or "gender" for that. I'm not defending all the people who are attention whoring by claiming fake genders, but I must defend the position that gender fluidity does exist, even though some people misuse that concept. I hope this clears up something (though I probably made it more confusing). I have a BA in Anthropology from NC State, so I"m not just speaking out my ass here.