Protozoans are single-celled organisms that belong to the kingdom Protista. They are typically microscopic and can be found in various environments, including freshwater, saltwater, and soil. Protozoans are diverse in shape and size, and they can move using structures like cilia, flagella, or pseudopodia. These organisms play essential roles in ecosystems, often serving as a food source for larger organisms and helping to decompose organic material. Some protozoans are beneficial, while others can cause diseases in humans and animals, such as malaria caused by the protozoan Plasmodium.