Taxonomy is the science of defining groups of biological organisms on the basis of shared characteristics and giving names to those groups. DNA, RNA and morphological characters are the main characteristics used to create modern phylogenies ("the tree of life") and define new species.
Interactive phylogenetic tree
"Tree of life by Haeckel" by Ernst Haeckel - First version from en.wikipedia; description page was here. Later versions derived from this scan, from the American Philosophical Society Museum.. Licensed under Public Domain via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:Tree_of_life_by_Haeckel.jpg#/media/File:Tree_of_life_by_Haeckel.jpg