When two organisms of different species live closely together, the relationship is known as a symbiotic relationship.
Mutualism
In a mutualistic relationship, both organisms living closely together benefit from one another.
Example:
Bees transport the pollen from flower to flower aiding in plant reproduction, while the flower provides nectar for the bee as a source of food.
Commensalism
In a relationship of commensalism, one of the organisms benefits, while the other organism is unaffected.
Example:
The moss growing on trees benefits from the location on a tree trunk, but the tree is unaffected.
Parasitism
In a relationship of parasitism, one organism benefits, while another organism is harmed.
Example:
When a leech takes blood from a human, the leech benefits and the human is harmed.