Protista is a diverse group of mostly single-celled organisms that belong to the kingdom Protista. They can be found in various environments, including freshwater, saltwater, and soil. This kingdom includes organisms such as amoebas, paramecia, and algae, which can perform photosynthesis or consume other organisms for energy. Protists are classified into three main categories: protozoa, which are animal-like and often mobile; algae, which are plant-like and can produce their own food; and fungus-like protists, which absorb nutrients from their surroundings. Despite their simplicity, protists play essential roles in ecosystems, such as being a food source for larger organisms.