Here's a breakdown of the terms:

* Microorganisms: This is a broad term for any organism that is too small to be seen with the naked eye, including bacteria, protozoa, algae, and fungi.

* Plankton: This term refers specifically to organisms that drift in water, unable to swim against currents. Plankton can be either plant-like (phytoplankton) or animal-like (zooplankton), and many single-celled organisms fall into this category.

So, while "microorganisms" is the most general term, "plankton" is a more specific term that applies to those microorganisms that drift in the water.