Taxonomy
Taxonomy is the science of identifying, describing, classifying and naming organisms, the various groupings referred as taxa (Singl., taxon). There are an enormous number of kinds of organisms described, and their study requires organization. Historically, organisms were classified according to their structure. Those with certain characteristics were put into one group, and those with different characteristics placed into another. These groups were divided and subdivided, producing a hierarchy of taxa, each with common structural characteristics according to a specified principle of classification. However, recent efforts have been made to reorganize some parts of the system based on the phylogeny of an organism-- that is, its relation to other organisms based on its evolutionary history.
Biologists group all organisms, living or dead, into one of three domains — Bacteria, Archaea, and Eukarya. In Eukarya, domains subdivide into Kingdoms. For example, the animal kingdom (Animalia) is divided into subgroups called phyla (e.g., Chordata), each of which (a phylum) is divided into classes (e.g., Mammalia), each class into orders (e.g., Carnivora), and each order into families (e.g., Felidae), and each family into genera (e.g., Panthera). Each genus (singular of genera) is divided into species (e.g., Panthera tigris, tigers). The species is the most basic category, the kind of animal, designated in a Latin binomial. The category of subspecies is sometimes used, often where the species doesn't satisfy the biological definition of species. That is, subspecies are usually organisms that have mostly identical characteristics and can interbreed, but live in different geographic locations.
In groups where there are many organisms, sometimes there are additional layers of classification: subclasses, suborders, superfamilies, subfamilies, and tribes.