Hierarchy of categories refers to an arrangement of different taxonomic groups in a definite order from higher to lower categories. A category is called a taxon. The main aim of hierarchical taxonomy is to assign an organism an appropriate place within the systematic framework of classification. The categories or taxa in the hierarchy are placed in ascending order. As we move upwards from species to kingdom, the level of similarities between the organisms decreases.

The taxa or categories used in classifying plants and animals are:

(i) Species: It is the lowest category that includes individuals with similar morphological characters and reproduces to form offsprings.

(ii) Genus: It includes related species which have less characteristics in common.

(iii) Family: Group of related genera are included in a family. More similar genus groups are placed together in a single family.

(iv) Order: Families resembling one another in few characteristics are assembled in an order.

(v) Class: It includes organisms of related order.

(vi) Phylum (in animals)/division (in plants): Different classes having few common characteristics are placed in a single phylum.

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