The three-domain system of classification is a biological framework that categorizes all forms of life into three primary domains: Bacteria, Archaea, and Eukarya.

This classification method, proposed by Carl Woese in 1990, is based on variations in genetic structure and cellular organization. It represents a significant advancement over earlier classification systems, such as the two-kingdom and five-kingdom systems. The three domains are as follows: Bacteria, Archaea, and Eukarya.

Both Bacteria and Archaea are classified as prokaryotes, which are single-celled organisms that lack a nucleus. Despite their similarities, they are differentiated by distinct characteristics in their ribosomal RNA (rRNA). Bacteria are known for their remarkable diversity and ubiquity, playing crucial roles in human health and disease. In contrast, Archaea thrive in extreme environments, such as hot springs and salt lakes.

The third domain, Eukarya, encompasses all organisms whose cells contain a nucleus. This domain is further subdivided into four kingdoms: Protista, Fungi, Plantae, and Animalia. Protists are predominantly single-celled organisms that do not fit into the categories of plants, animals, or fungi. The kingdom Fungi includes organisms like mushrooms, yeasts, and molds. Plantae consists of all plant life, ranging from mosses to towering trees. Lastly, Animalia includes all animals, from simple sponges to complex humans.

The three-domain system provides a more accurate representation of the evolutionary relationships among organisms compared to previous classification systems. It acknowledges the fundamental distinctions between the two groups of prokaryotes (Bacteria and Archaea) and the eukaryotes (Eukarya). This approach has significantly enhanced our understanding of the diversity and evolutionary history of life on Earth.