Plankton are a diverse group of organisms found in water or air that are unable to propel themselves against a current or wind. The word "plankton" comes from the Greek for "drifter" or "wanderer". Plankton are usually microscopic, often less than one inch in length, but they also include larger species like some crustaceans and jellyfish. Scientists classify plankton in several ways, including by size, type, and how long they spend drifting. But the most basic categories divide plankton into two groups: phytoplankton (plants) and zooplankton (animals) .

Phytoplankton are microscopic plants that perform photosynthesis to convert the suns rays into energy to support them, and they take in carbon dioxide and produce oxygen. Because they need the suns energy, phytoplankton are found near the waters surface. Zooplankton include microscopic animals, the young of larger invertebrates and fish, and weak swimmers like jellyfish. Most zooplankton eat phytoplankton, and most are, in turn, eaten by larger animals (or by each other) .

Plankton are the productive base of both marine and freshwater ecosystems, providing food for larger animals and indirectly for humans, whose fisheries depend upon plankton. As a human resource, plankton has only begun to be developed and exploited, in view of its high biological productivity and wide extent.