Mules are the offspring of a horse and a donkey. Horses and donkeys are different species, although they belong to the same genus, Equus, and are closely related enough to breed.
Mules are specifically the offspring of a female horse and a male donkey. If the cross is the other way, with a male horse and a female donkey, the offspring is called a hinny. Hinnies tend to take after their horse parent a little more, while mules usually look more like donkeys. Both mules and hinnies are usually sterile because donkeys and horses have different numbers of chromosomes.
Mules are often used for pack strings and other hard outdoor work because they tend to be more sure-footed and have better endurance than most horses. Their hooves are usually smaller and stronger, like donkey hooves, which makes them less prone to hoof problems and may allow them to travel without shoes even on rough ground. Mules are generally stronger than horses as well, which makes them excellent pack animals. They usually live up to a decade longer than horses and can remain in use throughout their lives.
Like horses and donkeys, mules come in all sizes, from miniature to giant. They can perform in almost any activity that horses do well in, although they are sometimes less successful in certain events that are based on appearance. Mules have a reputation for being more stubborn than horses, but this may be due to their intelligence.