The hot springs of Yellowstone National Park, USA, were among the first places Archaea were discovered. The Archaea are a group of single–celled microorganisms. A single individual or species from this domain is called an archaeon.
Archaea are sometimes referred to as extremophiles, inhabiting extreme environments (e.g., hot springs, salt ponds, Arctic ice, deep oil wells, acidic ponds that form near mines, and hydrothermal vents); however, these environments are not extreme to the archaea. In fact, many extremophiles die when moved to our environment.
The rest of the prokaryotes are classified as bacteria/ Eubacteria. Some textbooks and articles still refer to all prokaryotes as bacteria, but there is an increasing tendency to make a distinction between archaebacteria and bacteria.
Archaebacteria have no cell nucleus or any other membrane–bound organelles within their cells. Archaebacteria tend to live in extreme environments, this is possible due to their unique cell wall structures.
The cell wall of such bacteria is unique and consists of polysaccharides and proteins. The cell membranes of Archaebacteria contain branched–chain lipids, and therefore, they tolerate extremes of heat and pH.