Plankton is a collective term for all aquatic organisms that live in the water (not on the bottom) and cannot swim against the flow of water as fish can.
Plankton can be separated in many different groups, but the most basic ones are phytoplankton (plants/algae that get their energy from the sun) and zooplankton; plankton that gets its energy from eating other organisms or their remains.Some organisms live their whole life in the plankton, these are called holoplankton, while others are only planktonic in their larval stage, the meroplankton.
Plankton form the base of the food web in many aquatic ecosystems. Phytoplankton is eaten by zooplankton, which is eaten by other zooplankton and fish. It is thus a crucial component of ecosystems, and changes in the plankton can have big consequences.
The organisms in the plankton vary tremendously in size, shape and colour. Many bottom dwelling animals and fish have larval stages in the plankton with alien-like shapes and forms, looking completely different from the adult form.
Below you can find photos of actual plankton samples, showing what you can find in a single sample.