Archea:
microorganisms which are similar to bacteria in size and simplicity of structure but radically different in molecular organization. They are now believed to constitute an ancient group which is intermediate between the bacteria and eukaryotes.
For detailed explanation
Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebacteria kingdom), but this classification is outdated. Archaeal cells have unique properties separating them from the other two domains of life, Bacteria and Eukarya The Archaea are further divided into multiple recognized phyla. Classification is difficult because the majority have not been isolatede in the laboratory and have only been detected by analysis of their nucleic acid in samples from their environment.