Protists is a catch-all term used to describe all eukaryotic (cells with nuclei) organisms besides animals, plants, and fungi. Examples include the slime molds, water molds, the unicellular animal-like protozoa and amoeba and the plant-like protophyta. The term was abandoned as an official classification in 1990, being merged in with the fungi, plants, and animals to create the Eukarya "domain" of life, but it is still used frequently among biologists. The purpose of lumping in protists with the Eukarya domain is to emphasize that the differences between plants, animals, fungi, and protists is much less than the difference between the eukaryotes and the bacteria and archaebacteria.

The term "protist" derives from the Greek protiston, meaning the "first of all ones." Individual protists tend to be quite small, either unicellular or an undifferentiated multicellular mass. At one point, "Protista" encompassed everything that wasn't an animal or plant, until the advent of cellular biology which noted fundamental differences between bacteria and the rest of life. Now bacteria are a separate category from protists.

Protists acquire food by extending their cellular membrane around a food particle and absorbing it. Once within the cell wall, the protist forms a food vacuole around the particle and begins digesting it. Many protists eat bacteria for food. They are capable of paralyzing a living bacterium once they absorb it. A protist cell even has a tiny intestinal tract, around its Golgi apparatus.

Protists are broken down into three general groups: the animal-like protists, the plant-like protists, and the less numerous fungi-like protists. In the animal-like category, there are the flagellates, with their long flagella; the amoeboids, like the amoeba, able to form pseudopods at will; the ciliates, like the paramecium, covered in celia, and the spore-like sporozoa, immobile parasites such as Toxoplasma, which is believed to infect as much as 33% of the human species.

The familiar plant-like protists consist of the various forms of algae. These include green algae, red algae, and brown algae which includes the beautiful and numerous diatoms as well as kelp, which makes great forests in the sea. Of these, green algae are the most frequently encountered by humans.