1. Animalia: This kingdom includes all animals, from the smallest insects to the largest mammals. Animals are characterized by their ability to move, their heterotrophic mode of nutrition (meaning they get their food from other organisms), and their reproduction by sexual means.
2. Plantae: This kingdom includes all plants, from the smallest algae to the largest trees. Plants are characterized by their ability to produce their own food through the process of photosynthesis, their autotrophic mode of nutrition (meaning they do not need to eat other organisms to obtain their food), and their reproduction by both sexual and asexual means.
3. Protista: This kingdom includes all protists, which are a diverse group of organisms that do not fit into either the animal or plant kingdoms. Protists can be unicellular or multicellular, they can be autotrophic or heterotrophic, and they can reproduce by both sexual and asexual means.
This three-kingdom system was proposed by Ernst Haeckel in 1866 and was widely accepted until the 1960s when the five-kingdom system was introduced.