Mutualism- is a biological interaction between two organisms. Also in which one species benefits at the expense of the other.
Example: A clownfish and coral reefs. The clownfish benefit by having a protected home territory. But what does the anemoone gain from this?
Parasitism- a type of symbiotic relationship between 2 different organisms where one organism, the parasite, benefits at the expense of the host. In general the parasites are much smaller that their hosts.
Example: In mosquitos a female ingest blood for the protein. Male mosquitos ingest plant juices.
Commensalism- a type of relationship between 2 organisms in which one organism benefits and the other is unaffected.
Example: Barnacles getting attached to the skin of a whale or shell of a mollusk, the barnacle benefits by finding a home where nutrients are available, but the whale or mollusk shell isnt affected.