What is the difference between a donkey and a mule?

You Simply Asked and here is the answer:
- A male donkey or ass is called a jack, a female a jenny, and offspring less than one year old, a foal (male: colt, female filly).

While different species of the Equidae family can interbreed, offspring are almost always sterile. Nonetheless, horse/donkey hybrids are popular for their durability and vigor. A mule is the offspring of a jack (male donkey) and a mare (female horse). The much rarer successful mating of a male horse and a female donkey produces a hinny.

Thanks to our friends at Wikipedia - the free encyclopedia and also to Randall for his farmpertise (farm + expertise)

We now know that the first successful "hybrid" with the greatest fuel efficiency is the mule.  Ha!