Sex/ biological sex or Sex assigned at birth is what distinguishes an individual from another, based on the biological and physical organs of their body. A typical ‘male’ individual has a penis and testes, while a typical ‘female’ individual has a vagina and ovaries.
Gender as defined by WHO is “the socially constructed characteristics of women and men, such as gender norms, roles, and relationships of and between groups of women and men. It varies from society to society and can be changed.”
Over time the understanding of gender has changed. Gender is now understood as fluid, a spectrum, and a social construct.