Domains are the broadest categories in the classification of organisms. There are three known domains: archaea, bacteria and eukarya. Each of these is, in turn, subdivided into kingdoms, phyla, classes, orders, families, genus's and species.
Archaea is a highly diverse group of single-celled organisms that are abundant in nearly every habitat, including soil, wetlands, oceans, and even harsh environments that other organisms cannot tolerate.
The domain eukarya contains organisms whose bodies have cells that contain a nucleus and organelles and that are enclosed by plasma membranes. It includes four kingdoms: animals, plants, fungi, and protists. Protists include protozoa, slime molds and some algae. Humans are classified as eukarya, animals, mammals, primates, homo and sapiens.