Linnaeus (1707-1778) tried to fit every known organism into a single nested hierarchy. (In other words, he wanted to fit every organism onto a tree diagram.) The names (that we use today) for the levels of the "Linnean hierarchy" are:

   Phylum (plural phyla)
               Genus (plural genera)

So, for example, humans are in the family Hominidae, and our genus and species are Homo sapiens ("wise man").

(Latin was the language of science in the 1700's, and biological naming still follows that tradition.)

Fitting organisms into this scheme, or into any other scheme, is referred to as taxonomy.