Biological sex is the division of biological species into male and female varieties, each also known as a sex. Only some species demonstrate sex.
Humans demonstrate biological sex and sexual reproduction, specifically a form of binary anisogamy in which females (women) produce low quantities of large gametes (ova or eggs), whilst males (men) produce large quantities of smaller gametes (sperm). This is typical of other mammals as well.
Most humans fit into this binary classification of males and females. Intersex individuals exist but are rare (this of course makes them no less deserving of respect or compassion).
Biological sex may align with sociological gender, in which case the individual is cis-gendered. Or it may not, in which case the individual is trans-gendered.