Protozoa are single-celled organisms that belong to the kingdom Protista. They are diverse in form and function, often found in various environments, including soil, freshwater, and marine ecosystems. Protozoa play essential roles in nutrient cycling and serve as a food source for larger organisms. These microscopic creatures can be classified into several groups based on their movement and feeding methods. Some, like Amoeba, move using pseudopodia, while others, such as Paramecium, use cilia. Protozoa can be free-living or parasitic, with some species causing diseases in humans and animals.