Difference between protozoa, protists, protoctista?
These terms are all somewhat obsolete as taxonomic terms, but they may still be used to describe certain groups of organisms.

The term "protist" is used informally in two different ways.  One usage refers to the paraphyletic group consisting of all eukaryotes other than the animals, plants, and fungi.  The other usage is to refer to those eukaryotes that exist as independent cells or, if colonial, do not form colonies with differentiated tissues.

The term "protozoa" is used informally to refer to the "animal-like" protists, i.e. those that are non-photosynthetic and motile.

Wikipedia gives a good summary of how these terms were used historically.