The difference between a mule and an ass is that a mule is a cross breed or offspring of an ass and a female horse. The other name for ass is donkey and they are animals used to do heavy work on a farm. A mule is a sterile hybrid animal.
1 Additional Answer
The mule is a sterile, hybrid animal, the result of a donkey (ass) and a horse mare breeding. Mules are often used as pack animals while the ass or donkey is a species of the horse genus and often domesticated for work purporses