These are single-celled, eukaryotic organisms that are mainly parasitic in nature.
Amoeba, Paramoecium, Chlamydomonas, Micromonas, etc. are common protists.
Role of protists:
Protists form an important part of the food chain as they are a major source of phytoplankton.
Some protists are photosynthetic and hence produce oxygen.
Protists also form a section of decomposers that lead to the decomposition of biodegradable matter and help in increasing the organic content of the soil.