Taxonomic hierarchy is the system of arrangement of taxonomic categories in descending order. It was introduced by Linnaeus in 1751, who initially proposed five categories (class, order, genus, species, and variety).
However, the variety was obliterated and four new categories were introduced. Thus, the seven obligate categories in descending order are kingdom, phylum or division, class, order, family, genus, and species.
The placement of individuals or organisms in taxonomic categories is determined by their specific similar characters, dissimilarities, complexity, and evolutionary relationships.
Kingdom: This is the highest category in the taxonomy, includes all the organisms which share a set of distinguishing characters or include those phyla or divisions which are related at least in one character e.g. all animals in kingdom Animalia; all plants in the kingdom Plantae. Thus, maximum similarity occurs in species, this decreases with the increasing rank in the hierarchy.