Classification of living things

Linnaean taxonomy is a method of classifying living things, originally devised by (and named for) Carolus Linnaeus, although it has changed considerably since his time. The greatest innovation of Linnaeus, and still the most important aspect of this system, is the general use of binomial nomenclature, the combination of a genus name and a single specific epithet to uniquely identify each species of organism. For example, the human species is uniquely identified by the binomial Homo sapiens. No other species of organism can have this binomial. Prior to Linnaean taxonomy, animals were classified according to their mode of movement.
All species are classified in a ranked hierarchy, originally starting with kingdoms although domains have since been added as a rank above the kingdoms. Kingdoms are divided into phyla (singular: phylum) — for animals; the term division, used for plants and fungi, is equivalent to the rank of phylum (and the current International Code of Botanical Nomenclature allows the use of either term). Phyla (or divisions) are divided into classes, and they, in turn, into orders, families, genera (singular: genus), and species (singular: species).
Though the Linnaean system has proven robust, expansion of knowledge has led to an expansion of the number of hierarchical levels within the system, increasing the administrative requirements of the system (see, for example, ICZN), though it remains the only extant working classification system at present that enjoys universal scientific acceptance. Among the later subdivisions that have arisen are such entities as phyla, superclasses, superorders, infraorders, families, superfamilies and tribes. Many of these extra hierarchical levels tend to arise in disciplines such as entomology, whose subject matter is replete with species requiring classification. Any biological field that is species rich, or which is subject to a revision of the state of current knowledge concerning those species and their relationships to each other, will inevitably make use of the additional hierarchical levels, particularly when fossil forms are integrated into classifications originally designed for extant living organisms, and when newer taxonomic tools such as cladistics and phylogenetic nomenclature are applied to facilitate this.
There are ranks below species: in zoology, subspecies and morph; in botany, variety (varietas) and form (forma). Many botanists now use "subspecies" instead of "variety" although the two are not, strictly speaking, of equivalent rank, and "form" has largely fallen out of use.
Groups of organisms at any of these ranks are called taxa (singular: taxon) or taxonomic groups.