Taxonomy is the branch of biology concerned with three interlinked processes: identifying organisms, naming them, and classifying them into groups based on shared characteristics. It rests on the idea that classification is easiest when organisms are arranged hierarchically, from very broad groups down to very precise ones. A closely related term, systematics, covers the same ground but also considers the evolutionary relationships among organisms.
Nomenclature is only useful if a name is universally understood, so scientific naming follows fixed international rules — the International Code of Botanical Nomenclature (ICBN) for plants and the International Code of Zoological Nomenclature (ICZN) for animals. Carolus Linnaeus introduced the system in use today, binomial nomenclature, in which every scientific name has two parts: a generic name (the genus) and a specific epithet (the species). By convention, biological names are treated as Latin regardless of their origin and are printed in italics, or underlined separately if handwritten; the genus name begins with a capital letter and the specific epithet is written entirely in lowercase, even when it commemorates a person or place. The name of the author who first validly described the species may follow in abbreviated form, as in Mangifera indica Linn.
Classifying an organism is not a single step but a hierarchy of ranks, each called a category or taxon (plural taxa), running from the most inclusive to the most precise: kingdom, phylum (division, for plants), class, order, family, genus and species. Species is the most basic category — a group of actually or potentially interbreeding individuals sharing fundamental similarities. A genus groups together closely related species that share more features with each other than with species placed in other genera — lion, tiger and leopard are three species united in the genus Panthera. Related genera are grouped into a family, related families into an order, related orders into a class, and related classes into a phylum or division; all phyla or divisions together make up a kingdom. Moving up this hierarchy from species to kingdom, each successive category contains progressively more organisms that share progressively fewer characteristics.
A specific epithet can recur across entirely unrelated genera, so it never identifies a species by itself — indica names the mango, Mangifera indica, and equally the neem, Azadirachta indica, two plants that are not closely related at all. The full binomial, genus plus epithet together, is what actually specifies the organism.