The term "protozoa" refers to a group of single-celled eukaryotes.
Ciliates are a category of protozoans that are distinguished by the occurrence of cilia, which are hair-like organelles that are structurally related to eukaryotic flagella but are shorter, more numerous, and have a different oscillating pattern than flagella.
Ciliated protozoans include Vorticella and Paramecium.
Protozoa can be as small as 1 micrometre and as large as several centimetres or more.
Free-living protozoa are common and abundant in both fresh and salt water, as well as other moist environments such as soils and mosses.