Plankton refers to a diverse group of microscopic organisms that drift in aquatic environments, including both freshwater and marine ecosystems. They are classified into phytoplankton (plant-like organisms that perform photosynthesis) and zooplankton (tiny animals that feed on phytoplankton or other small organisms). Plankton play a crucial role in the food web, serving as a primary food source for many larger organisms, including fish and whales. Their abundance and diversity are vital for maintaining ecological balance in aquatic habitats.