Biology vs. Culture. Sex refers to the biological and physiological characteristics that define who is a man and who is a woman. Gender is more of a social construct and refers to the activities, roles that culturally acceptable/appropriate for men and women. Thus, gender aspects will vary from society to society and from culture to culture (e.g. In some societies women are not allowed to wear pants "man's" clothing).
There is not one. Those two terms are synonymous.