According to molecular analysis, scientists have grouped all living things into three domains. Eukarya, Bacteria, and Archaea. The domain Eukarya includes protists, fungi, plants and animals. The Bacteria are the larger of the two domains of prokaryotes and include a wide range of organisms with different life styles.
They live in fresh water, salt water, on land, and even in the human intestines. Bacteria are usually surrounded by a cell wall that protects them and determines their shape. Their cell wall contains a carbohydrate-rich substance called peptidoglycan, which makes them different from archaea which lack peptidoglycan.
The archaea are equally small as bacteria, they lack nuclei and have a cell wall that is chemically different from the cell wall of bacteria. They also have different membrane lipids comprised of ethers as opposed to ester lipids of bacterial and eukaryotic cell membranes. Also, the DNA sequences of key archaeal genes are more like those of eukaryotes. Archaea live in extremely harsh conditions and environments such as acid lakes or volcanic hot springs.