Kingdom Protista belongs to the domain Eukarya. This domain is characterized by organisms that have complex cells with a nucleus and organelles, unlike prokaryotic organisms which lack a defined nucleus.
Protista is a diverse group of mainly unicellular organisms that can be autotrophic (like algae) or heterotrophic (like protozoa). The classification of Protista within the domain Eukarya highlights the evolutionary relationships among eukaryotic organisms. While this kingdom is often viewed as a catch-all category for eukaryotes that do not fit neatly into plants, animals, or fungi, it plays an important role in ecological systems and biological research.