It should be noted that as a classification unit, there is only one animal kingdom, Animalia. Fungus belongs to the kingdom Fungi and plant belongs to the kingdom Plantae.

In biologist's term, animals are a monophyletic group, which means every animal on Earth descended from one common ancestor. As a result, there are many similarities among animals.

All animals are multicellular eukaryotes, which means that they are composed of many cells and each cell has a nucleus when that cell is first formed. The nucleus is a membrane bound organelle that contains all the genetic material of the cell. Animal cell also does not have a cell wall.

Animals also share six thousand groups of genes, among which 25 groups are exclusive to animals. This is the basis that biologists determine that all animals shared a common ancestor 650 million years ago.

The animal kingdom is a diverse and ancient group. As a consequence, besides those basic cellular and genomic structures, there are few truly universal traits. The following applies to many, but not all, animals:

- Generally, animal cannot produce their own nutrients and must consume other organisms for subsistence.
- Generally, animal requires oxygen for respiration.
- Generally, animal can move spontaneously.
- Generally, animal cell differentiate into specialized cell type to better serve the organism.