Symbiosis is the relationship of two or more organisms living and interacting closely. In order for it to be mutualism, both organism have to benefit. For example, the bacteria in your intestines help you digest your food. Without them you couldn’t survive and without you they couldn’t survive.
When one organism is helped and the other is harmed it is called parasitism. An example of parasitism would be a flea on a cat. The flea feeds on the blood of the cat, because of the loss of the blood the cat is harmed. Another example of paratism would be a tick and a deer. The tick does the same as the flea. The deer is harmed in the same way except that the tick can be poisonous.
When one of the organisms benefits, and the other is not harmed, it is called commensalism. When commensalim is found in the organs it is called endocommensalism. When it is found on the surface of the skin, it is called ectocommensalism.