Metazoans are animals. Metazoa is the scientific name for the group of multicellular, eukaryotic organisms in the kingdom Animalia, and today the two names are treated as synonyms. The term dates to 1874, when Ernst Haeckel split the animal kingdom into the multicellular Metazoa and the single-celled Protozoa, a grouping that is no longer used for single-celled life.
Metazoans eat organic material, breathe oxygen, have muscle cells, can reproduce sexually, and develop from a hollow ball of cells called a blastula; with few exceptions, they can move. The group runs from sponges, comb jellies, placozoans and cnidarians such as jellyfish to the bilaterians, which include insects, molluscs, worms and vertebrates. More than 1.5 million living animal species have been described.
Source: Wikipedia, Animal
Image: Animal diversity collage by Justin, CC BY-SA 3.0, via Wikimedia Commons.