Gender refers to the social and cultural roles, behaviors, and expectations that society assigns to individuals based on their sex [sex: the biological differences between males and females]. For example, in many societies, women are expected to be nurturing and take care of the household, while men are expected to be strong and provide for the family.
Very Simple Definition:
Gender is how society expects people to behave based on whether they are a boy or a girl. For instance, girls are often expected to wear dresses and play with dolls, while boys are expected to wear pants and play with cars.