theres alot of things that make up plankton.
holoplankton- that spend there entire life cycle as a part of plankton( most algea,copepods,salps, and jellyfish)
then theres
meroplankton,- which are only planktonic for part of there lives, usually a larval stage like the larvae of sea urchins,starfish,crustacians,wormsand most fish.
phytoplankton- diatoms,cyanobacteria,and dinoflagellates
zooplankton- small protezoa that feed of off plankton
bacterioplankton- whick is well bacteria
in terms of size theres
megaplankton - 20 to 200 cm (metazoans)
macroplankton- 2-20 cm (pterpods)
mesoplankton-.2mm-2cm (copepods)
microplankton-20-200um (protists)
nanoplankton-2-20um (small protists
picoplankton - .2-2um(bacteria)
femtoplankton-less then .2 um (marine virusis)
hope this helps (sorry about the spelling)