Commensalism: One organism benefits and the other is unaffected (no loss no gain)
Examples: epiphytic plants in trees; algae on turtle's shell
Mutualism: Both organisms benefit.
Example: ants and ant plants [plants feed & house the ants; ants protect the tree]
Parasitism: One organism benefits the other organism is harmed.
Example: heartworms in a dog's heart and fleas on a dog's skin.
Predation: One organism eats another organism
Examples: deer eats grass; mountain lion eats deer
Competition: One organism competes with another for the same resources (The fitness of one is lowered by the presence of the other, ie. survival of the fittest)
Examples: Two male elk compete for a female; Cheetas and lions compete for the same prey