The Archea, though prokaryotes like bacteria, are distinct in several ways:

1.  Archea lack peptidoglycan.  With the exception of the mycoplasmas,
     most eubacteria synthesize peptidoglycan.

2.  The cell membrane of the archebacterial cell is composed of branching lipid
     chains held together by ether (hydrocarbon) bonds rather than ester (lipid) bonds.

3.  When synthesizing new proteins, the first amino acid coded by archeal mRNA is
     methionine, rather than N-formylmethionine used by eubacteria.

4.  Though most archebacteria live in moderate conditions, many can live in extreme
     environments.

Extremophiles

Geogemma (a above) and Pyrodictium (b) are both examples of thermophiles.
 


This hot spring in Yellowstone National Park shows an example of a thermophile
bloom.

Halophiles such as Halobacterium salinarium grow in salinity between 17% and 35%.  Most use bacteriorhodopsins rather than bacteriochlorophylls to drive their process of photosynthesis.

Methanogens are obligate anaerobes that utilize CO2, H2 and organic acids, producing methane (CH4) as waste.  Many are members of the normal flora of animals.  Methanogens in the gut of a cow can produce up to 400 liters of CH4 a day.