Here's a breakdown:
* Multicellular: They are composed of many cells that work together to form tissues, organs, and organ systems.
* Eukaryotic: Their cells have a nucleus that contains their genetic material (DNA). They also have other membrane-bound organelles within their cells, like mitochondria.
While there are many other common characteristics, like movement and heterotrophy (consuming other organisms for energy), being multicellular eukaryotes is the most fundamental characteristic that defines all animals.