The Protoctista are a varied collection of eukaryotic organisms — mostly single-celled and aquatic — that don't fit the fungi, plants or animals. It's the kingdom for "everything else with a nucleus".
1 · What unites them
Members of Protoctista are eukaryotic (true nucleus + membrane-bound organelles) and mostly unicellular, though some — like the large seaweeds — are multicellular. Most live in water or damp places. Because they are so diverse, the kingdom is often called a "ragbag" — defined more by what its members are not than by what they share.
2 · Three informal groups
Animal-like (protozoa) — heterotrophic, often motile. e.g. Amoeba (moves and feeds with pseudopodia), Paramecium (covered in beating cilia), and Plasmodium (the malaria parasite).
Plant-like (algae) — autotrophic, photosynthetic. e.g. Euglena, diatoms, Spirogyra and the seaweeds.
Fungus-like — e.g. the slime moulds, which feed on decaying matter.
Euglena is famous for sitting on the fence: it photosynthesises like a plant and can take in food like an animal.
3 · Moving and feeding
Protozoa move in three classic ways: by pseudopodia ("false feet", Amoeba), by cilia (tiny beating hairs, Paramecium) or by flagella (long whips, Euglena). Amoeba also feeds by surrounding food with pseudopodia and engulfing it (phagocytosis) into a food vacuole.
4 · Reproduction & importance
Most reproduce asexually by binary fission; some also have sexual stages. Their importance is huge:
Why they matter Photosynthetic protists (phytoplankton, algae) form the base of aquatic food chains and produce a large share of Earth's oxygen. But some cause serious disease — Plasmodium causes malaria, and Entamoeba causes amoebic dysentery.
In one minute
Protoctista = eukaryotic, mostly unicellular & aquatic; a "ragbag" kingdom.