Three-domain system

The three domain system of biological classification was introduced by Carl
Woese to reflect his discovery that the prokaryotes comprise two very
different groups of organisms. In it, all living things are grouped into
three domains. Two of these, the Bacteria and Archaea (originally Eubacteria
and Archaebacteria), include prokaryotes. The third, the Eukarya or
Eukaryota, includes all eukaryotes, including the older kingdoms Animalia,
Plantae, Fungi, and Protista.

These domains are typically divided into kingdoms. Other ranks higher than
kingdom had been earlier, for instance empires, to group together the
eukaryotes, but in these schemes the prokaryotes were retained as a single
group, understood to be paraphyletic. Some have argued that such systems are
actually preferrable, that is, that the differences between Bacteria and
Archaea are not sufficient to warrant such a level of separate treatment.

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