A mule is a domestic equine hybrid between a donkey and a horse. It is the offspring of a male donkey (a jack) and a female horse (a mare). The horse and the donkey are different species, with different numbers of chromosomes. Of the two possible first-generation hybrids between them, the mule is easier to obtain and more common than the hinny, which is the offspring of a female donkey (a jenny) and a male horse (a stallion). Mules vary widely in size and may be of any color. They are more patient, hardier, and longer-lived than horses, and are perceived as less obstinate and more intelligent than donkeys.

Source: Wikipedia