The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archæon (sometimes spelled "archeon"). In the past they were viewed as an unusual group of bacteria and named archaebacteria but since the Archaea show many differences in their biochemistry from other forms of life, classifying the Archaea is difficult.
There are three phyla, or groups, of archaebacteria. The methanogens are characterized by their ability to harvest energy by converting H2 and CO2 into methane gas. They (and all archaebacteria) are obligate anaerobes, meaning they cannot live in the presence of oxygen. They are found in marshes and in the intestinal tracts of humans and some animals (cows, for example).
The second phylum is the extreme halophiles. These organisms are names because they are salt-loving. Though salt kills most bacteria, it helps the extreme halophiles to thrive. They are found in the Dead Sea, the Great Salt Lake, and other areas with a high salt content.
The third phylum is the thermoacidophiles. These bacteria are found in extremely acidic conditions and in areas with very high temperatures. They can survive in areas with temperatures as high as 230 degrees Fahrenheit and with pHs below 2 (hydrochloric acid, which is incredibly strong, has a pH of 1). These locations include volcanic vents and hydrothermal vents (cracks in the ocean floor where scalding water leaks out).