Plankton are aquatic drifting organisms which means they are incapable of swimming against the current and instead, "go with the flow." The term, plankton, doesn't define the organism but how the organism lives. Plankton may be plant-like organisms, called phytoplankton or animal-like organisms, called zooplankton.
Plankton can be producers, consumers or both. They play important roles in aquatic food webs and are a source of food for larger aquatic organisms.