World Health Organization: The word gender is used to describe the characteristics, roles and responsibilities of women and men, boys and girls, which are socially constructed. Gender is related to how we are perceived and expected to think and act as women and men because of the way society is organized, not because of our biological differences. 
Health Canada: Gender refers to the array of socially constructed roles and relationships, personality traits, attitudes, behaviours, values, relative power and influence that society ascribes to the two sexes on a differential basis. Gender is relational - gender roles and characteristics do not exist in isolation, but are defined in relation to one another and through the relationships between women and men, girls and boys. 
Sex is our biology, everything else is gender. "If you know that the difference is 100% biological it’s a sex difference. Everything else must be considered a gender difference."