The Y chromosome is a sex chromosome that when present in most animal means the animal would be classified as male. Males have one X chromosome and one Y chromosome (heterogametic); whereas females have two X chromosomes (homogametic). The Y chromosome is also much smaller then that of the X chromosome, and therefore contains less genes, leading to X-linked disorders. Although in moths, butterflies, some fish and most birds it is the female that is heterogametic. So there is no confusion when writing about these genetic differences, the nomenclature of ZZ/ZW (instead of XX/XY).