Hierarchical system of classification was proposed by Linnaeus. In this classification, he arranged organisms into an ascending series of increasing inclusiveness forms. The taxonomic groups are arranged in a definite order, from higher to lower categories. Each category is called a taxon.

The taxa used in classifying organisms are:

(i) Species: It is the lowest category that includes individuals with similar morphological characteristics, which are able to breed among themselves and reproduce. For example, horse (Equus cabalus) and ass (Equus asinus) belong to the same genera Equus but are different species.

(ii) Genus: It includes related species which have less characteristics in common. Members of a genus have identical reproductive organs. For example, a banyan and a fig tree differ from each other in vegetative structures but resemble each other in reproductive organs, and thus they belong to the same genus Ficus.

(iii) Family: Group of related genera are included in a family. More similar genera are placed together in a single family. For example, a cat and a lion belong to the same family Felidae. They both possess similar structures and retractable claws.

(iv) Order: Families resembling one another in few characteristics are assembled in an order. For example, a tiger and a wolf share characteristics such as jaws with powerful incisors, large, sharp canines and carnivores. Thus they both belong to the same order Carnivora.

(vi) Phylum (in animals)/division (in plants): Different classes having few common characteristics are placed in a single phylum. For example, all animals which have a notochord in the embryo belong to phylum chordata.

(vii) Kingdom: Organisms that share a set of distinguishing common characteristics are placed in the same kingdom.

Example: Plant kingdom, Animal kingdom, etc.