"what is the difference between organisms described as plankton, nekton, and benthos? "

Plankton are small aquatic organisms that cannot move on their own. They live in the photic zone. They include phytoplankton and and zooplankton 

Nekton are aquatic animals that can move on their own by swimming through the water. The can live the photic or aphotic zone and feed on plankton or other nekton. Some examples are fish or shrimp

Benthos are aquatic organisms that crawl in sediments in the benthic zone. The benthic zone is the deepest part of a body of water. Most of the organisms that life here include sponges, clams, and anglerfish