I've mentioned mules before, but I just checked and I haven't fully discussed what a mule actually is.
A mule is the offspring of a mare (female horse) and a jack (male donkey).
Mules have long ears like donkeys and full tails like horses, with a build that falls somewhere between the two. They have scraggly manes (which are generally shaved off).
Many people consider mules superior to horses. They live longer (in England, the phrase "donkey's years" for a long time refers to the long lifespan of burros), are more surefooted, eat about one-third less per day and need a lot less water. (I have personally ridden a mule for several hours in a desert environment without watering it, something I would never dream of doing with a horse). They're also less likely to panic and bolt.
However, mules are slower than horses and have a reputation for being stubborn. My personal, limited experience is that mules are less subtle than horses and will really let you know what they think. They don't have a work ethic that will...okay, let's put it bluntly. Horses will tolerate almost anything you do to them. Mules just won't take your crap.
A horse has 64 chromosomes and a donkey 62. Because of this, mules end up with 63 chromosomes. This renders mules sterile. However, john (male) mules are always castrated early as they can be extremely hard to handle otherwise.
Or more accurately, mules are mostly sterile. There have been a few documented instances of molly mules giving birth, with the most famous being Old Beck who produced several foals including Pat Murphy, Jr., who appeared to be pure horse and had normal fertility. Most recently, in 2007, a molly gave birth unexpectedly on a Denver ranch to an animal that resembled a mule.
(To note, some of the reported cases of fertile mollies have turned out to be something else - molly mules are infertile but have normal maternal instincts and equines of all species are notorious for trying to steal other people's foals).
Four saddle mules picketed at Indian Garden in the Grand Canyon. This image doesn't give a good view of their size - the three larger mules are "Missouri mules" which are bred from Belgian mares and American Mammoth Jack stallions (the American Mammoth Jack is an extremely large donkey breed that is related to large donkey breeds found in Spain and France, all of which are perpetuated entirely to breed big mules). The smaller mule on the end is probably from a Quarter Horse mare. The brown and sorrel mules in the center each stand over 16 hands.
Mules are colored similarly to horses, but normally have extreme mealy/pangare, as seen on the sorrel mule. When they lack this, per the brown mule, it's so unusual there's a specific term for it - such mules are called "blue mules."