The "Three Domains" is a biological classification system that categorizes all living organisms into three major groups: Bacteria, Archaea, and Eukarya. This system was proposed by Carl Woese in the 1990s and is based on genetic and molecular differences. Bacteria are single-celled organisms without a nucleus, while Archaea are also single-celled but have distinct biochemistry and genetics, often thriving in extreme environments. Eukarya includes more complex organisms, such as plants, animals, and fungi, which have cells with a nucleus. This classification helps scientists understand the diversity of life on Earth.