Mules are passive mobs that were introduced to Minecraft in Update 1.6. In survival mode, they cannot be found naturally either. They can only be obtained through the breeding of a Horse and a Donkey.
In order to breed a mule, one must have a donkey and a horse of any kind. To breed them, feed each a Golden Carrot or Golden Apple which will activate love mode. They will then run to each other, and a baby mule will be born between them.
Mules are slightly smaller than their horse parent, and when they grow they share traits of both a donkey (the long ears), and the horse (color, tail, etc.).
To ride a mule, first a player must domesticate or "tame" it. Once tamed, riding is as simple as right clicking the mule and no longer requiring an empty hand. At first the mule will allow the user to ride it, and it will wander around aimlessly. To control the movement, a Saddle must be equipped. In order to do so a player must open their Inventory while riding the mule and placing a saddle in the appropriate saddle slot on the mule's inventory.
Mules have the same usage as donkeys do. They do not allow the use of Horse Armor, but they can carry items for a player. A mule can only hold one Chest even though it appears to have two chests (one on each side). A chest is equipped by simply right-clicking a mule with the chest in hand.
Like horses, some mules are faster while others can jump higher, and still others have more health. The fastest mule is slower than the fastest possible horse. The mule is a combination of the horse and donkey with the appearance of a horse with long ears.
One may ask what advantage mules have over donkeys since they both carry chests. There are a few reasons. First, the mule has a certain color, like donkeys, that is always the same, but it is different from the color of donkeys. The second feature is that they inherit traits from the horse parent as well while donkeys have a fixed speed and jump height. This can be exploited by breeding a quick or jumpy horse with a donkey which can give the newborn mule better speed or jump height than a donkey depending on the horse's speed and jump height.