Mules are a cross between a donkey and a horse. These two species are very, very closely related. When you refer to a living thing by its scientific name, everyone else in the world, whatever their mother tongue, will know exactly which organism you are talking about, even though it will have a different common name. He worked out the system of naming them which has been used ever since. He used the Latin language, because scholarly people all over the world knew Latin.
Scientists still use Latin for naming living things.
He grouped his specimens according to their similarities and then sub-divided each group progressively into smaller groups until he ended up with one single type that couldn't be subdivided any more. These he called a species, and all the individuals could interbreed. The species all had closely related cousins and all these cousins belonged to a larger group which he called a genus. Linnaeus was married and had lots of children. When he died, without leaving much money to bring up the children, his wife sold the collection of specimens to a biological society in London.