When two organisms of different species live closely together, the relationship is known as a symbiotic relationship.
In a mutualistic relationship, both organisms living closely together benefit from one another.
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.
In a relationship of commensalism, one of the organisms benefits, while the other organism is unaffected.
The moss growing on trees benefits from the location on a tree trunk, but the tree is unaffected.
In a relationship of parasitism, one organism benefits, while another organism is harmed.
When a leech takes blood from a human, the leech benefits and the human is harmed.