Sex refers to the biological and the anatomical difference between females and males.
Gender refers to the culturally and socially constructed differences between females and males found in the meanings, beliefs, and practices associated with male and female. "Femininity" and masculinity" it is important to distinguish between the two because they are very different and people seem to use them "interchangeably".
In order to properly distinguish between these two terms society need to know exactly what they represent and their implications. Too often people use these terms incorrectly so it is very important that they understand the difference between the two terms .
Primary sex difference between females and males are the genitals used in the reproductive process. Males carry an X and Y chromosome and females carry two X chromosomes etc. Secondary differences are the physical traits that identify an individual's sex. Not including the reproductive system there traits includes the fact that men are usually taller than women, and females tend to have lighter skeletons, different shoulder-pelvis proportions etc .
Sex isn't always clear cut between male and females there are cases where a person is born with both male and female genitals. This person is referred to as a hermaphrodite. There are also cases of transsexuals. They are ordinary people who believe that he or she was born with the body of the wrong sex. In western societies there are males who live as woman or females who live as a man but do not alter the genitalia. This person is referred to as a transvestite.
In society individuals learn gender roles, which are the "attitudes, behaviors, and activities that are socially defined as appropriate for the each sex and are learned through the socialization process". Gender identity is a person referring to the self as male or female. On a larger scale the analysis of gender examines the structural features, external to the individual that continues to cause gender inequality.