Competition: Two or more organisms vying for the same food source or energy source. Competition can be inter-specific (between individuals of different species) or intra-specific (between individuals of the same species).
Predation: One organism (the predator) kills and consumes another (the prey) for food (energy).
Parasitism: One organism (the parasite) takes food (energy) from another (the host) without killing it.
Mutualism: A relationship involving two organisms in which both organisms benefit.
Commensalism: An ecological relationship between two organisms in which one benefits and the other is not affected.