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:
Kingdom
Phylum (plural phyla)
Class
Order
Family
Genus (plural genera)
Species
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.