The XY sex-determination system is a sex-determination system present in many mammals, some insects, some snakes, some fish, and some plants.
In this system, the sex of an individual usually is determined by a pair of sex chromosomes. Typically, females have two of the same kind of sex chromosome (XX), and are called the homogametic sex. Males typically have two different kinds of sex chromosomes, and are called the heterogametic sex. From Wikipedia
