Final answer:
Protists are a diverse collection of eukaryotic organisms not classified as animals, plants, or fungi, with size ranging from microscopic to multicellular giant seaweeds. They play various roles in ecosystems and some cause human diseases. 'Protist' remains a useful term despite new classification insights.
Step-by-step explanation:
Protists are a diverse group of eukaryotic organisms that are classified neither as animals, plants, nor fungi. This group includes a wide variety of organisms, ranging from unicellular entities such as algae and plankton to multicellular forms like the giant seaweeds. Unique characteristics of protists are their simple organization and the absence of specialized tissues found in more complex eukaryotic forms. Protists inhabit environments from soil to freshwater and marine ecosystems, playing critical roles as primary producers and also as agents of human diseases like malaria and sleeping sickness.
While historically these organisms were grouped into the Kingdom Protista, molecular genetics has revealed that some protists are more closely related to animals, plants, or fungi, leading to a restructuring of their classification. However, the term 'protist' persists as a useful, informal way of referring to this incredibly diverse array of eukaryotes that extends well beyond single-celled or microscopic lifeforms, thus debunking the statement that all protists are microscopic single-celled organisms.