Plants and animals are placed in a series or succession of different ranks or categories according to their natural relationship. Hierarchy is a system in which members of a type are organized in ranks one above the other. Heierarchy of categories or taxonomic hierarchy is the system of arrangement of taxonomic categories in a descending order depending upon their relative dimensions. It was introduced by Linnaeus. And is therefore also called as Linnaean hierarchy or taxonomic hierarchy.
Linnaeus proposed 5 categories- class, order, genus, species and variety. Later workers deleted the category of variety and added the categories of kingdom, phylum, and family. The taxonomic categories which are always used in hierarchical classification of organisms are called obligate categories. They are seven in number. Obligate categories in the descending order are kingdom, Phylum, class, order, family, genus and species.
Species: The lowest taxon or category in the biological classification is species. So the basic unit of taxonomy is species. Species are a group of individuals which can interbreed in the natural environment and produce fertile offspring.
Genus: A genus is a group of related species. All the species in a genus has many common characteristics and all have evolved from a common ancestor.
Family: A family is an assemblage of related genera.
Order: An order is a category of related families.
Class: Several related orders having certain common characters form a higher category called the class.
Phylum or Division: Classes having same features in common constitute a phylum.
Kingdom: The highest taxon or category in the biological classification is the Kingdom. It includes one or more related division or Phyla.