The living world consists of millions of species of organisms. These present enormous diversity ranging from micro-organisms to the highest evolved plants and animals. The knowledge about all these organisms will be highly confusing, meaningless and useless if they are not properly identified and arranged systematically.

The systematic arrangement of properly identified and named organisms is called classification, systematics or taxonomy. (taxis = arrangement, nomos = order or law)

Thus, taxonomy is a branch of biology which deals with collection of organisms, their identification, nomenclature and systematic grouping or classification into various categories. This is done on the basis of similarities and differences of their morphological, anatomical, cytological, genetical, physiological, biochemical, developmental and other characteristics.

The similarities of characteristics between species or groups of species indicate their relationship. This is also gives us some idea about their phylogeny (i.e. their evolutionary history).

The classification of plants into various groups is called plant taxonomy or systematic botany. Similarly, classification of animals is called animal taxonomy or systematic zoology.

(1) Taxa (Singular: Taxon): A taxon is the taxonomic group of any rank in the system of classification (H.J.Lam, 1948). For example, in plant kingdom, each one of the following such as, angiosperms, dicotyledons, polypetalae, Malvaceae, Hibiscus esculentus, etc. represents a taxonomic group i.e. a taxon. A taxon may be a very large group such as a Division (e.g. angiosperms), or it can be a very small group such as a species (e.g. Hibiscus esculentus).

(2) Categories (Singular: Category): In the system of classification, the various taxa are assigned definite ranks or positions according to their taxonomic status. Each such taxonomic rank is called the taxonomic category. The various major categories in the classification of plant kingdom are Kingdom, Division (Phylum), Class, Series, Order, Family, Genus and Species.

Arranging various taxonomic categories in their proper order on the basis of their taxonomic ranks is called taxonomic hierarchy (systematic hierarchy). In this hierarchy, the kingdom represents the category of highest rank while the species is the category of the basic rank.

Following is an example of the taxonomic hierarchy representing the methodology of classifying a plant and an animal in a scientific manner.

The system of giving a scientific name to each properly identified plant or animal is called nomenclature.

A system of nomenclature of plants and animals in which each scientific name consists of two parts or sub-names is called the system of binomial nomenclature.

Thus according to this system the scientific name of sunflower is Helianthus annuus and that of man is Homo sapiens. In the above names, the first part of the name (i.e. Helianthus or Homo) represents the name of the genus (generic name). The second part of the name (i.e. annuus or sapiens) represents the name of the species (specific name).

This system of binomial nomenclature was introduced by Carolus Linnaeus in 1753 in his book Species Plantarum.

The system follows certain rules, such as :

1. The scientific name must be in Greek or Latin language.
2. Genetic name should come first and must begin with a capital letter.
3. The same name should not be used for two or more species under the same genus.
4. The scientific name must be either underlined or written in italics.
5. The name of the author who first described the species should be written after the specific name (e.g. Homo sapiens Linnaeus).

Carolus Van Linnaeus : (1707-1778) Carolus Linnaeus is popularly known as the father of taxonomy, because of his remarkably outstanding contributions to the field of systematics. Linnaeus was a great Swedish naturalist. He studied at the University of Lund and later worked in the University of Uppsala. He described nearly 6000 plant species and 4000 animal species from different parts of the world. Linnaeus published a number of books on taxonomy. The most notable among these are Systema Naturae (1735) and Genera Plantarum (1737) which contain the artificial system of classification of angiosperms based on the sexual characters (e.g. characters of stamens). His book Species Plantarum (1753) is a landmark in the history of taxonomy. It contains the binomial system of nomenclature which has been practiced ever since.