Protozoa are single-celled eukaryotes (organisms whose cells have nuclei) that commonly show characteristics usually associated with animals, most notably mobility and heterotrophy.
They are often grouped in the kingdom Protista together with the plant-like algae and fungus-like water molds and slime molds.
In some newer schemes, however, most algae are classified in the kingdoms Plantae and Chromista, and in such cases the remaining forms may be classified as a kingdom Protozoa.
The name is misleading, since they are not animals (with the possible exception of the Myxozoa).
Protozoa have traditionally been divided on the basis of locomotion.
Most protozoans are too small to be seen with the naked eye - most are around 0.01-0.05 mm, although forms up to 0.5 mm are still fairly common - but can easily be found under a microscope.