Plankton (singular plankter) are any organisms that live in the water column and are incapable of swimming against a current.  They provide a crucial source of food to many large aquatic organisms, such as fish and whales.

These organisms include drifting animals, plants, archaea, algae, or bacteria that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification. (From Wikipedia)

Though many planktic (or planktonic—see the next section below) species are microscopic in size, plankton includes organisms covering a wide range of sizes, including large organisms such as jellyfish.