Protista are a kingdom of organisms that include all eukaryotes (organisms with a cell nucleus) that are not classified as plants, animals, or fungi. Protists include algae, slime molds, and protozoans, and can be found in a variety of habitats including soil, fresh and salt water, and as parasites of other organisms. Many protists are single celled organisms, but some like giant kelp can form huge structures.
Protists can have significant economic impact either as a result of having useful properties such as a variety of marine algae (seaweeds), or as a result of pathogenic behaviour that causes harm to humans or economic crops. Some protista live in symbiotic relationships with other organisms such as a variety of protozoa that can be found in cows' rumens, or algae that live in a symbiotic relationship with fungi to form lichens.