Like most scientists, biologists need to stay organization to study organisms effectively. Taxonomy uses with a naming system to classify like animals and their features. Taxonomy made a breakthrough in the 18th century with a biologist named Carolus Linnaeus.
Linnaeus popularized the binomial nomenclature formal naming system that involves the use of a two-part Latin name -- a capitalized genus followed by a species. For example "Equus cabbalus" is the proper binomial nomenclature name for a horse. "Equus" is the genus and "cabbalus" is the species. The International Congress on Zoologists adopted formal rules for the system and solidified its usage. Under the system, every animal that humans are aware exists has a zoological classification and binomial nomenclature-based scientific name.
In Latin, "species" means "kind." In science, species is a group of interbreeding organisms that do not usually breed with other species. Although about 1.5 million species have been identified, biologists estimate that between 10 and 40 million actually live on Earth.
Linnaeus also developed a systematic hierarchy of taxa (plural taxon) to categorize all plants and animals. From broadest taxa to most narrow, the taxa are: kingdom, phylum, class, order, family, genus and species. Each taxa is slightly less broad than the one previous to it, with species being the most restrictive and classifying the fewest number of organisms. The field of botany adopted the system in 1753, and zoology followed in 1758.
Just as the classification system broadens the number of organisms it encompasses as it goes higher than genus and species, the system also further narrows as it gets lower. Below genus and species, an animal can be further divided into a subspecies, variety, and subvariety or form. These subdivisions use a similar naming system called trinomial nomenclature, or three-part name.