Burros, Donkeys, and Mules

I don't know how to physically tell the difference between burros, donkeys, and mules.  Never really had a need to know.  However, on one of my walks I saw a whole field full of them (this picture is only a few) at my neighbor's place.  I had heard braying off and on since last week but guessed it to be the wind through some piece of equipment set up at the gravel pit.  I don't know if these guys are just staying temporarily or if my neighbor will be keeping them.  Need to plug into the local gossip to find out if I need to learn how to tell an ass.  (No, save the cutesy comments.)