Plankton is not a single animal it is the group of organisms that are not able to move against the current of water and they wander in the sea similar to something that fly freely in the direction of air.
These organisms are mostly the bacteria, krill, algae and larva's of sea animals like fishes . Thousands of fish larva can be found in the collection of these organisms and they are used as a source of food for other sea animals.
Krill can be found in abundance as million of them are found in sea water. As bacteria and algae are small in size so you can found million of bacteria in one teaspoon of water. Bacteria and Eukaryotic plants are part of the nano-plankton. Another class of these animals is called Phytoplankton which can make there own food through photosynthesis. They have the ability to convert the carbon dioxide and sunlight into edible gold.
They have many advantages that's why they are necessary for maintaining the ecosystem of the sea. They also produce oxygen which is also essential for most of the sea species.