Sex=male/female, a fact of biology determined at conception inside your mother's body.

Gender, previously known as sex stereotypes =masculine/feminine, cultural concepts learned and enforced through socialization after birth.

There's also a distinction between sex roles related to reproduction and sex roles that are based on sex stereotypes rather than biology, aka gender roles.

Sex roles mean women, not men, are ones who carry pregnancies, go through labor, give birth and breastfeed. Women take on these roles coz of biology. Many women wouldn't mind sharing these roles with men - or letting men do them altogether. But that's not possible coz of how nature has arranged things. So these roles go to women alone.

Sex roles based on sex stereotypes, aka gender roles, come from ideas rather than biology, though many of the ideas can be traced back to biology. Gender roles are based on reasoning such as: because women breastfeed babies, women are the ones who "naturally" should do all the cooking & grocery shopping in families/couples; because women breastfeed babies, women should be held responsible for dealing with the mess when what has been fed to babies comes out the other end; because women carry, give birth to and breastfeed babies, women should do all the childcare as children grow up, even long long after they've stopped breastfeeding ... This kind of thinking leads in turn to further iterations like: because it's women's "natural role" to change babies' diapers & wipe their asses, it's women's job to clean house, do the laundry, including ironing, wash the dishes and scrub the toilet; because women are "naturally suited" to cleaning house, then they're the ones who are best equipped to work as housekeepers in hotels; because women are "naturally suited" to wiping baby's shitty asses, women are naturally suited to changing bedpans in hospitals and to changing the diapers and wiping the asses of elderly incontinent people; because women do the bulk of childcare in most families, women are best suited to being teachers and administrators in early-childhood education, but not to being college professors and university administrators and presidents ... and so on.