The Archaea are a group of prokaryotic and single-celled microorganisms. In this they're similar to bacteria but these two groups evolved differently, and are classified as different domains in the three-domain system. Originally these organisms were named archaebacteria. However, this term hasn't been favored since the three-domain system became popular. Although there's still uncertainty in the phylogeny, Archaea, Eukaryota and Bacteria were introduced as the fundamental classifications in w… (More on Archaea)