Classification involves hierarchy of steps, in which each step represents a rank or category, called taxonomic category or taxon. All taxa together constitute a taxonomic hierarchy as follows:
Species $\rightarrow$ Genus $\rightarrow$ Family $\rightarrow$ Order $\rightarrow$ Class $\rightarrow$ Phylum or division $\rightarrow$ Kingdom.
Most common characters among individual members are found in taxon species. Common characters decrease from species to kingdom and members of a kingdom have least number of common characters. Similarly, complexity of classification decreases from species to kingdom