a mule is usually a character you use to just hold stuff. there's also various skill mules in which case people lvl the character just to be able to use a certain skill, like powerguard, or holy symbol, or hyperbody. some people also have monster carnival mules and such to just do cpq over and over again for the necklace, or to sell free wins.
in real life a mule is a beast of burden. and is also a term designated for people who are used to smuggle illegal items across borders. but I don't think that's what you were asking =p
Mule is a character made for a certain single purpose.
Like creating a character for a certain skill.
A "mule" to earn money by selling high and buying low.
A CPQmule is a character to replace a person on the other team, trying to get free wins non-stop.
I hope that summarizes it.