The three domains of life are:

• Archaea : These are prokaryotic organisms without a nucleus but with unique biochemical and genetic features distinct from bacteria. Archaea often live in extreme environments such as high salt, high temperature, or acidic conditions. They have unique membrane lipids and lack peptidoglycan in their cell walls. Their RNA and genetic transcription mechanisms are more similar to eukaryotes than to bacteria

• Bacteria : Also prokaryotic and lacking a nucleus, bacteria have different membrane lipids than archaea and typically contain peptidoglycan in their cell walls. They include many familiar microorganisms, some of which are pathogenic to humans. Bacteria are more extensively studied due to their abundance and ease of cultivation

• Eukarya : These organisms have eukaryotic cells characterized by a true nucleus and membrane-bound organelles. This domain includes all plants, animals, fungi, and protists. Eukarya are distinct from the prokaryotic domains Archaea and Bacteria

This three-domain system was introduced by Carl Woese and colleagues in 1990 based on differences in 16S rRNA gene sequences, reflecting fundamental evolutionary divisions among cellular life forms

. It supersedes earlier classification systems by recognizing Archaea as distinct from Bacteria despite both being prokaryotic. In summary, the three domains are:

• Archaea (prokaryotes with unique biochemistry)
• Bacteria (prokaryotes with typical bacterial features)
• Eukarya (organisms with eukaryotic cells)