Archaea are single celled organisms that have a cellular structure very similar to that of bacteria. They were earlier classified as a species of bacteria and called archaebacteria but have now been given a separate class of their own.
Though archaea are found in all types of environments, a large number of them are extremophiles that live in environments with extreme conditions like very high temperatures, high levels of acidity or alkalinity, high salinity, etc. that other organisms find very difficult to survive in.
For example, the group of archaea called halophiles lives in extremely saline environments. Thermophiles flourish in hot springs where temperatures are above 45 degree Celsius with hyperthermophiles existing in temperatures as high as 80 degree Celsius and some even above the boiling point of water. Acidophiles exist only in extremely acidic environments, a notable example being a species named Picrophilus torridus that can survive in pure sulfuric acid.