* Bacteria: These are single-celled organisms that lack a nucleus and other membrane-bound organelles. They have diverse metabolisms and are found in nearly every environment on Earth.

* Archaea: These are also single-celled organisms lacking a nucleus and other membrane-bound organelles. They are often found in extreme environments (like hot springs or very salty lakes), but they are also found in more moderate environments.

While both groups are prokaryotes, archaea are distinct from bacteria in their genetics, biochemistry, and cell wall structure. They are more closely related to eukaryotes (like plants and animals) than they are to bacteria.