In modern biology, there are three approaches to classifying organisms: systematics, cladistics and molecular evolutionary taxonomy. They are all based on organisms' relation to each other, but use different indicators to assign the degree of relationship.
Modern taxonomy, or classification system, originated in the 18th century, from the works of a Swedish botanist Carolus Linnaeus. He classified living beings based on similarities between them. Organisms which could interbreed were put in one species. There are levels of organization above species: genus, to which a number of closely-related species belong, a family, which consists of related genera, and order, which includes similar families. Class, phylum and kingdom are the three subsequent top levels of the system. These large groups can include sub-groups, for example, subphylum, or be a part of a supergroup, for example, a superclass.
When Carolus Linnaeus was developing his system, evolution had not been a scientific fact yet. Once scientists started studying how different organisms are related to each other based on a common ancestor they share, classification has also moved on. The phylogenetic classification system, or systematics, lists clades of organisms, organized into right-angled diagrams, which have a common ancestor. In cladistics, the separation is made at the point when a trait, which makes a particular species unique, arises. It can be, for example, upright walking for humans. A similar system of molecular evolutionary taxonomy focuses on the emergence of genetic differences between species.