Animals are eukaryotic and usually multicellular (although see Myxozoa), which separates them from bacteria and most protists. They are heterotrophic, generally digesting food in an internal chamber, which separates them from plants and algae. They are also distinguished from plants, algae, and fungi by lacking cell walls. All animals are motile,if only at certain life stages. In most animals, embryos pass through a blastula stage, which is a characteristic exclusive to animals.

The seven essential functions of animals are feeding, respiration, circulation, excretion, response, movement, and reproduction. These functions carried out by an animal allow them to maintain homeostasis.

Unlike simple animals, complex animals have a higher level of cephalization (having distinct heads), bilateral symmetry, cell specialization (instructed to perform a specific task), and higher organization of organ systems.

Echinoderms and other vertebrates are similar in the sense that they are all deuterostomes, meaning their anus emerged from blastopore. This piece of information suggests that they are cousins in evolutionary sense.

Most animals that exhibit bilateral symmetry have their sense organs and nerve cells concentrated at the front of their bodies. This characteristic is called cephalization, which forms the head of an animal. If an animal exhibits body symmetry and cephalization, it can detect stimuli in their environment more quickly.

The feeling of hunger is triggered when the body releases hormones to replenish the nutrients that the body need for cellular activities. When the demand of nutrients is met, the feeling of hunger is gone. This is an example of internal homeostasis.