Protozoa are single-celled organisms that belong to the kingdom Protista. They are microscopic and can be found in various environments, including soil, freshwater, and saltwater. Protozoa can be classified into different groups based on their movement, such as amoebas, which move using pseudopodia, and flagellates, which use whip-like structures called flagella. These organisms play essential roles in ecosystems, serving as both predators and prey. Some protozoa are beneficial, helping to decompose organic material, while others can cause diseases in humans and animals, such as malaria caused by the protozoan Plasmodium. Understanding protozoa is crucial for studying ecology and health.