Taxonomy

Classification is the province of the disciplines of systematics and taxonomy. Taxonomy places organisms in groups called taxa, while systematics seeks to define their relationships with each other. This classification technique has evolved to reflect advances in cladistics and genetics, shifting the focus from physical similarities and shared characteristics to phylogenetics.
Traditionally, living things have been divided into five kingdoms:

• Monera --
• Protista --
• Fungi --
• Plantae --
• Animalia
  However, many scientists now consider this five-kingdom system to be outdated. Modern alternative classification systems generally begin with the three-domain system:
• Archaea (originally Archaebacteria) --
• Bacteria (originally Eubacteria) --
• Eukarya
  These domains reflect whether the cells have nuclei or not, as well as differences in the cell exteriors.
  Further, each kingdom is broken down continuously until each species is separately classified:
  
• Kingdom
• Phylum
• Class
• Order
• Family
• Genus
• Species
  The scientific name of an organism is obtained from its genus and species. For example, humans would be listed as Homo sapiens. Homo would be the genus and sapiens is the species. Whenever writing the scientific name of an organism, it is proper to capitalize the first letter in the genus and put all of the species in lowercase; in addition the entire term would be put in italics or underlined. The term used for classification is called taxonomy.

There is also a series of intracellular parasites that are progressively "less alive" in terms of metabolic activity:
Viruses -- Viroids -- Prions
The dominant classification system is called Linnaean taxonomy, which includes ranks and binomial nomenclature. How organisms are named is governed by international agreements such as the International Code of Botanical Nomenclature (ICBN), the International Code of Zoological Nomenclature (ICZN), and the International Code of Nomenclature of Bacteria (ICNB). A fourth Draft BioCode was published in 1997 in an attempt to standardize naming in these three areas, but it has yet to be formally adopted. The Virus International Code of Virus Classification and Nomenclature (ICVCN) remains outside the BioCode.