One of two groups into which plankton are divided, the other being phytoplankton. Zooplankton are a large group of micro- and macroscopic animals ranging in size from a fraction of a millimeter to 30-50 millimeters, with a few, such a certain jellyfish, being up to a meter in diameter. Some plankton, called permanent plankton or holoplankton, are adapted to a pelagic mode of existence and remain floating or feebly swimming throughout their entire life cycle. Others, called temporary plankton, are the transitory floating stages such as eggs, larvae, and juveniles of the benthos and nekton. This latter category is usually seasonal in occurrence and the abundance is primarily neritic since it derives from the benthos and nekton of shallow areas.

According to Rigby and Milsom (2000):

Members of the zooplankton have the widest geographical spread and greatest numerical abundance of any animals. Modern zooplankton are important contributors to global biomass and to the chemistry of the oceans, a dominant means of flux to the seabed, and a source of food for many large animals. The microzooplankton are dominated by flagellate protists, including some dinoflagellates and zooflagellates, and by amoebae such as foraminifera and radiolarians. Planktic ciliates are common, although the major group of these, the tintinnids, have proteinaceous tests and leave little record in the sediment. The macrozooplankton include a wide range of solitary and colonial cnidarians, chaetognath and polychaete worms, and holoplanktic gastropods. Crustaceans are among the most common macrozooplankton, with copepods, euphausiids, amphipods, ostracodes, and decapods all abundant and diverse. Urochordates are widespread with two planktic groups, appendicularians and salps. Larval stages of invertebrates and fish make up a significant proportion of the heterotrophic plankton in the modern ocean, remaining as part of the plankton for periods ranging from minutes to years.