Protozoa, from Greek "first life", are small (but not simple) organisms. They are single-celled heterotrophic eukaryotes, which eat bacteria and other protists.
They are Protista, and are classified in a number of different phyla. Both 'Protozoa' and 'Protista' are not monophyletic (not single clades), but rather convenient hold-alls.
Protozoa are of many different kinds (e.g. Amoeba, Paramecium). Some protozoa are parasites (disease-causing germs).