They include some weird bacteria that live in very salty places, like the Dead Sea or the Great Salt Lake and bacteria that live in hot springs. The Archaea are very different in many ways from Eubacteria and other cells. One of the ways in which they are different is with respect to the composition of the cell membranes. The cell membranes of the Archaea have a lipid structure with a hydrophobic interior and hydrophilic surfaces, as in other organisms. But the lipids are quite unusual.
Some lipids even span the membrane, with hydrophilic groups on each end of a hydrophobic molecule.
This sort of structure is found in no other types of cells. It seems that the Archaea biochemistry may have a lot of distinctive features. The Archaea are classified in four distinct phylogenetic groups. They must live in an oxygen-free environment. They use hydrogen gas as an energy source and produce methane.