Some authors name this group the Kingdom Protoctista. The rationale is that traditionally the term Protista was used for unicellular organisms only. Because this kingdom includes multicellular organisms in addition to unicellular organisms, many individuals prefer to use a term not traditionally associated with unicellular organisms, hence the term Protoctista. I have no preference for either term, but because most of the textbooks available for introductory general biology and plant biology use the term Protista, we use it in this set of on-line readings.
The Kingdom Protista is a diverse group of eukaryotes that are either heterotrophic or photosynthetically autotrophic. Some protists are unicellular, others are simple colonies of cells, while others are fairly complex colonies.
Protozoa are members of the Protista that are unicellular, animal-like organisms. When there were only two kingdoms used for classification, protozoa were defined as single-celled animals. Protozoa do not have cell walls and are heterotrophic: like animals, protozoa ingest food instead of absorbing it like the fungi. Common examples of protozoa include Amoeba, Paramecium, and Trichomonas.
Algae are either unicellular or colonial Protista that have plant-like characteristics, the most notable being photosynthesis and cell walls that contain cellulose. Many botany texts still include the algae in the plant kingdom because plants generally are considered to have evolved from one of the groups of algae. Algae are divided into subgroups based upon their pigmentation. Common algal groups include green, golden-brown, brown, and red algae. Representative well-known algae include Chlamydomonas (green), kelp (brown), diatoms (golden-brown), and Spirogyra (green).