Answer:
Plankton are mostly tiny animal and plant organisms that float or weakly swim in the ocean's surface waters. Some plankton are single-celled while others are multi-celled---with some forming colonies. The word plankton comes from the Greek planktos, meaning drifting or wandering.
The organisms that make up plankton are very numerous and diverse; they can be grouped in many different ways. One division is based on the ability for photosynthesis; those plankton (mostly algae) that are capable of this process are called phytoplankton. Non-photosynthesizing (often termed animal) plankton are called zooplankton. The divisions of plankton are not made based on plant or animal characteristics---because some plankton can exhibit traits of both plants and animals.