The formal evolutionary hierarchy of groups of organisms proceeds from the largest to the smallest groups: domain – kingdom – phylum – order – class – family – genus – species. Living organisms are grouped according to the type of cells they consist of, either prokaryotic cells or eukaryotic cells. Prokaryotes have a simple internal architecture without a nucleus. Eukaryotes have a distinct internal structure with a nucleus containing the genetic material. A third group of living organismswas recognized in the late 1960s, the Archaea (also called archaebacteria). They differ from ordinary bacteria by their plasma membrane (isoprene ether lipids rather than fatty acid ester lipids) and lifestyle. They are assigned to two classes, Crenarchaeota and Euryarcheota.
Source : Thieme Color Atlas of Genetic 3ed (2007)