Different steps or categories of taxonomy together is known as hierarchy. Hierarchy may be of two types, which are described below :- (1) Linnaean Hierarchy: Linnaeus (1707-1778) first introduced five steps or categories for animal classification and this is known as Linnaean Hierarchy. These categories are-Kingdom, Class, Order, Genus and Species. Scientists later introduced seven categories in Linnaean hierarchy for animal classification these are-Kingdom, Phylum, Class, Order, Family, Genus and Species. (ii) Modern Hierarchy: Simpson (1945) expanded the steps of hierarchy by prefixing "Super", "Sub' and 'Infra' with the normal categories and introduced 21 steps for detailed classification of animals. This is known as Modern Hierarchy.

Keys are used to identify different species. Typically, a key will ask questions based on an organism's easily identifiable traits. Questions with only two possible responses are used in dichotomous keys. They might be displayed as a table of questions or as a tree of questions with branches.