Holozoa is a group of living things that includes animals, and some single-celled relatives of animals. It excludes fungi. Holozoa is also an old name for the tunicate genus Distaplia.
Holozoa is a clade: a group of plants or animals with a common ancestor. It has all organisms close to animals.
An example of a well-known holozoan is the choanoflagellate, which looks a lot like the cells of a sponge. Proterospongia is an example of a choanoflagellate that might give clues as to how sponges evolved.