This is a relationship between members of two different species. What makes this relationship special is that both species benefit from being together. Symbiotic species live closely together and provide services for each other. A species in a such a relationship is called a symbiont.
An example of a real symbiotic relationship can be found in the fictional movie Finding Nemo. Nemo the clownfish lived in a sea anemone. Sea anemones often have clownfish living inside their swaying tentacles. Those tentacles are filled with a nasty venom. Their sting keeps most clownfish-hunting predators at bay. In return for protection, the clownfish protects the sea anemone from anemone-seeking predators. The clownfish also nibbles parasites off the tentacles. The clownfish and anemone can live without the other, but they do better together.