The key characteristics all animals share are multicellularity, eukaryotic cells and heterotrophy, according to About.com. If it weren't for sponges, cells that are organized into tissues would mark another similarity between all animals. Sexual reproduction is another characteristic shared by most, but not all, animals.
Regardless of species, all animals share multicellularity, which means their bodies consist of multiple cells. This sets animals apart from organisms, such as single-celled algae, fungi, bacteria and other basic life forms. Most plants are also multicellular, so although this is a characteristic shared by all animals, it is not one unique to animals.
Every animal on the planet is a eukaryote. A eukaryote is an organism that consists of cells that have membrane-bound nuclei and organelles. All animals are eukaryotes, but other organisms, such as some plants and fungi, are eukaryotes as well. The opposite of a eukaryote is a prokaryote, an organism that does not have complex cells.
Finally, all animals are heterotrophs. A heterotroph is an organism that ingests the carbon in other organisms, such as plants and animals, to sustain its life. An organism that obtains its carbon by absorbing it from its environment, such as a plant or fungus, is called an autotroph.