Classification involves hierarchy of steps in which each step represents taxonomic category. Each taxonomic category is referred to as a unit of classification and is commonly termed as taxon (pi.: taxa).
Taxonomic hierarchy :
Taxonomic hierarchy is a system of arranging all taxonomic categories in descending order with kingdom at the top and species at the base.
As we go up the taxonomical hierarchy, the number of individuals increases but the number of common characteristics goes on decreasing.