A mule is the offspring of a male donkey and a female horse.
A hinny is the opposite - bred from a stallion (male horse) on a jennet/jenny (female donkey).
Hinnies are not always possible to distinguish from mules - if in doubt, the animal is referred to as a mule. They're also considerably rarer because it is, for reasons that are presumably to do with the chromosome numbers, much harder to breed the cross successfully. Mule breeders will generally only make a hinny if they really want a particular cross.
Hinnies tend to look a little bit more like horses than mules do. Anecdotally, some muleskinners claim that you can tell whether an animal is a mule or a hinny by turning them out in a field which has a couple of horses and a couple of donkeys in it - a mule will go hang out with the horses and a hinny will seek out the donkeys.
Mule ears. Just because.