Answer: The three domains of life are Bacteria, Archaea, and Eukarya.
Carl Woese et al. in 1977 described these three domains of life under the biological classification. According to this classification, all the cellular life forms can be categorized into one of these three domains based upon their differences in the nucleotide sequences in the ribosomal RNAs (rRNA), lipid structure of the cell's membrane, and antibiotic sensitivity.
- The Archaea- It is also called as  Archaebacteria. It is the oldest domain and constitutes the most primitive forms of bacteria. The organisms live in extreme environmental conditions. Example- Methanogens, which produce methane gas.
- The Bacteria- It is also called as Eubacteria. Most of the pathogenic prokaryotic organisms come under this domain. Example- Spirochaetes which causes syphilis and Lyme disease.
Archaea and Bacteria are single-celled microorganisms which come under prokaryotes ( no well defined nucleus or cell organelles).
- The Eukarya- It is also called as Eukaryotes ( well defined nucleus and cell organelles). It comprises organisms like Plants, animals, fungi, and protists.
