There are 3 different types of symbiotic relationships. Parasitism occurs when a smaller organism lives off of a larger one and gives nothing in return. Leeches are parasites. Many viruses and bacteria are parasites. Because the parasite depends on its host it rarely kills it.
Mutualism occurs when both organisms benefit.
Such as when algae lives in the gut of a protozoan. Commensalism occurs when one organism benefits and the other is unaffected. Such as when water mites and flatworms live in the canals of freshwater sponges.