Close living associations are called symbiotic relationships or symbiosis. Three types of symbiosis are commensalism, mutualism and parasitism.
Commensalism
Commensalism is the interaction between two organisms where one benefits and the other is unharmed. An example of commensalism is nitrogen-fixing bacteria, Rhizobium living in the nodules of leguminous plants convert atmospheric nitrogen into nitrates which are taken up by the plants to make plant proteins.
Mutualism
Mutualism is the interaction between two organisms where both benefit. An example of mutualism is lichen which composed of alga and fungus. The fungus depends on the photosynthetic algae for food, the algae depends on the fungus for anchorage, moisture and nutrients.
Parasitism
Parasitism is the interaction between two organisms where the parasite benefits and the host is harmed. An example of parasitism is tapeworms (parasites) which reside in human body (host).