Marine and freshwater organisms, which, because they are nonmotile or because they are too small or too weak to swim against the current, exist in a drifting, floating state. The term plankton is a collective name for all such organisms and includes certain algae, bacteria, protozoans, crustaceans, mollusks, and coelenterates, as well as representatives from almost every other phylum of animals. Plankton is distinguished from nekton, which is composed of strong-swimming animals, and from benthos, which include sessile, creeping, and burrowing organisms on the seafloor. Large floating seaweeds (for example, Sargassum, which constitutes the Sargasso Sea) and various related multicellular algae are not considered plankton but pleuston. Organisms resting or swimming on the surface film of the water are called neuston (e.g., the alga Ochromonas).
Source: Encyclopaedia Britannica 2002