Parasitism: when one organism benefits by harming another organism. An example would be mistletoe attaching itself into a desert plant.

Predation: when a heterotroph preys on and ultimately eats another heterotrophic organism. An example would be a coyote and a roadrunner.

Competition: when two organisms fight for the same food source. An example is eagles and rattlesnakes competing for birds to eat.

Relationships

Commensalism: when one organism benefits from an action that doesn't effect the other one. An example is a cactus wren builds its nest in a cactus to protect its young from predators such as raven. There is no harm to the cactus.

Mutualism: when both organisms benefit from an action. An example is Bees and moths pollinating cacti flowers.