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Domains
Domains are the newest taxonomic division of life. All life on Earth, both modern and ancient, falls into one of these three immense categories. Archaea, Bacteria and Eukarya (sometimes called Eukaryota) make up the three domains. The division between two of the domains remain less precise due to lateral gene transfer, the movement of environmental DNA into and out of the smallest and simplest organisms. Still, Woese's three-domain classification system is becoming the most accepted classification as it most accurately reflects the sweeping and fundamental differences between these types of life.
Archaea
Creatures in the domain Archaea are prokaryotes; they are unicellular and have no nuclei or membrane-bound organelles, the tiny structures within other types of cells. Although they initially appeared similar to their fellow prokaryotes in the domain Bacteria, organisms in the Archaea domain have a distinct biochemistry that sets them apart. Bacteria and Archaea exhibit so many differences in their function and structure that Woese and other scientists believe that they may have a distinct evolutionary history from other prokaryotes. Organisms that fall into this domain include extremophiles -- organisms that thrive in high salinity, temperature or acidity -- and methanogens that produce methane.
Bacteria
The other prokaryotes form the domain Bacteria. Scientists once termed them "eubacteria," or "true bacteria"; now that classification is one of the kingdoms within the greater domain. Biologist and author Stephen Jay Gould referred to bacteria as "the root of life's entire tree" in part because they shaped the planet as it is today via photosynthesis. Cyanobacteria, the first photosynthetic organisms, thrived millions of years before plant life arose. As essential as cyanobacteria were for life, other members of the domain have more sinister reputations; virtually all disease-causing prokaryotes are bacteria including the organisms responsible for plague, anthrax and staphylococcal infections.
Eukarya
All living creatures that do not fall into the two prokaryotic domains are members of the domain Eukarya. All plants, animals and fungi have more in common with one another than with Archaea or Bacteria and therefore share this domain. The name Eukarya comes from Greek words meaning "true seed;" it describes the nucleated cells and membrane-bound organelle structure that all members of Eukarya share. While not all Eukarya are multicellular, all multicellular organisms are Eukarya.