It is an old term, and the wider term protist is generally preferred today. However, 'protozoa' is often used for convenience, especially in junior education. It is a rather convenient hold-all term, but actually 'protozoa' are classified in a number of different phyla.
The term is problematic because modern biochemical and genetic techniques show that it includes several quite different kinds of cells. They are found only in moist or aquatic habitats. Many protozoan species are symbionts, some are parasites, and some are predators of soil bacteria and algae. There are about 30,000 protozoan species.
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Protozoa Facts for Kids. Kiddle Encyclopedia.