Taxonomy is the science of classification of organisms. Biologists classify organisms based on their evolutionary relationships, using a hierarchical system of grouping by shared features.
The fundamental unit of life on Earth is what biologists refer to as the species. Although the concept of a natural unit for classification is critical to many fields within the biological sciences, biologists are far from agreeing on its exact definition, and the meaning of a species may vary somewhat depending on the type of biologist you question. For our purposes, the widely accepted "biological species concept" is quite appropriate. The biological species concept states that a species is a population or series of populations whose members are able to interbreed freely under natural conditions and who do not breed with other species. Although there are exceptions to and difficulties with this definition, it works for most organisms. A simpler way to define species, which is not complete but will work for young audiences, is that a species is a unit of classification that refers to a population (group) or series of populations (group) of closely related and similar organisms.
Closely related species are combined into a genus, while closely related genera form a family, and so on up the classification hierarchy. This classification hierarchy, listed from the largest or most inclusive category, is shown below. Taxa for the white-footed mouse are listed in the right-hand column as an example. Note that the genus and species names are always either italicized or underlined, with the genus name capitalized and the species name given in lower case.
Phylum (animal) or Division (plants) ...... Chordata
subphylum ................ Vertebrata
Class ... Mammalia
Order .. Rodentia
Family . Muridae
Genus .. Peromyscus