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 convenient hold-all term, and most systems of taxonomy classify 'protozoa' into several 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. About 30,000 protozoan species are known.