A plankton net is a device that can be used to catch plankton (little plants and animals that live in the ocean). Scientists use special plankton nets with high volumes, but anyone can make a plankton net by tying a leg pantyhose to the end of a metallic ring with a handle. This can be dipped into the water from a slow-moving boat to collect plankton for observation under a microscope. Keep dragging the plankton net until it is obvious that there are many little bits of organic material caught in it. That's the plankton.
Plankton exist everywhere in the pelagic (illuminated) zone of the ocean, but are at relatively low concentrations in most places, requiring a plankton net to concentrate them for observation. The plankton you are likely to catch will be small, generally 2 mm in diameter or less, so you'll need a magnifying glass, or, better yet, a microscope to view them. Plankton only lives for a couple days after you remove it from the ocean with a plankton net, so you'd better look at it fast. It can be preserved for slightly longer by putting it in the refrigerator. If you want to preserve the planktonic organisms for a while after they die, add alcohol to the seawater so it becomes 70% alcohol.
Some of the organisms you may find using a plankton net include copepods (1-2 mm crustaceans), amphipods (slightly larger shrimp-like crustaceans), tunicates (sac-like filter feeders), cladocerans ("water fleas"), ostracods ("seed shrimp," 0.2 to 30 mm wide), chaetognaths ("arrow worms," about 10 mm), and bits of algae. If you have a more powerful microscope, you can see the smallest planktonic organisms, including rotifers and various larvae of larger animals.
Plankton engage in bust-and-boom cycles, where their numbers increase rapidly in warmer weather and drop in cooler weather. This is because their ultimate food source, the algae, is very sensitive to environmental conditions. The most numerous plankton is in the tropics, where the temperature and nutrient levels are ideal.