Life on Earth is divided among three domains, or large groupings. Two of these, Archaea and Bacteria, are comprised exclusively of microbes. The third group, Eukaryota, contains both multicellular and microbial life, according to the University of California Museum of Paleontology. This classification system developed in response to a dawning awareness that much of life's diversity is confined to microbes.
Bacteria are among the oldest living things on Earth. Fittingly, this group is also one of the most genetically and morphologically diverse. Some bacteria are destructive to plants and animals, while others live in symbiosis with them. Others are important commercially or as the base of the food chain, as reported by UCMP.
Archaea are some of the hardiest organisms on Earth. This domain is home to the so-called extremophiles that make their living in the superheated water of hot springs as well as hypersaline lakes and deep-sea seeps. Archaea are also abundant among the ocean's plankton. According to UCMP, Archaeans' affinity for extreme environments makes them difficult to culture in the lab.
The third major classification for life is Eukaryota. Eukaryotes are the group that includes all animals, plants and fungi. The group also includes microbes known as protists. This group includes the amoebae and radiolaria.