Protista, or protists, are a diverse group of eukaryotic microorganisms that are primarily unicellular but can also include multicellular forms. They belong to the kingdom Protista and encompass a wide range of organisms, such as algae, amoebas, and protozoa. Protists can be autotrophic, heterotrophic, or mixotrophic, allowing them to thrive in various environments, including freshwater, saltwater, and soil. This kingdom is crucial for ecological balance, playing roles in primary production and nutrient cycling.