The three major domains of life are Bacteria, Archaea, and Eukarya. Bacteria are unicellular organisms characterized by their prokaryotic cell structure, lacking a defined nucleus. Archaea, also prokaryotic, are known for thriving in extreme environments and have distinct biochemical processes. Eukarya encompasses all organisms with eukaryotic cells, including plants, animals, fungi, and protists. This domain classification reflects fundamental genetic and evolutionary differences, providing a framework for understanding the diversity of life on Earth.