There are two genders in humans: male and female. Biologically, gender is determined by a person's chromosomes and results in different body types. The main anatomic gender differences are the genitals.
In all societies, gender also has an important influence on social roles. Minor males are called boys while minor females are called girls. Adult males are called men while adult females are called women.
Usually adult males are sexually attracted only to adult females and vice versa. This is called heterosexuality. Some individuals are sexually attacted to members of their own gender; this is called homosexuality.
Some people, collectively called transgender, don't identify with their biological gender but (usually) with the opposite gender, with both genders, or none.