The Archaea (or Archea) are a group of single-celled organisms.The name comes from Greek αρχαία, old ones. They are a major division of living organisms.Archaea are tiny, simple organisms.he Archaea (or Archea) are a group of single-celled organisms. The name comes from Greek αρχαία, old ones. They are a major division of living organisms. Archaea are tiny, simple organisms.