mutualism: "A mutualistic relationship is when two organisms of different species "work together," each benefiting from the relationship. One example of a mutualistic relationship is that of the oxpecke and the rhinoceros or zebra. Oxpeckers land on rhinos or zebras and eat ticks and other parasites that live on their skin. The oxpeckers get food and the beasts get pest control. Also, when there is danger, the oxpeckers fly upward and scream a warning, which helps the symbiont (a name for the other partner in a relationship)."

bird+zebra.jpg
http://www.necsi.edu/projects/evolution/co-evolution/mutualistic/co-evolution_mutualistic.html

Commensalism:

"Barnacles adhering to the skin of a whale or shell of a mollusk: Barnacles are crustaceans whose adults are sedentary. The motile larvae find a suitable surface and then undergo a metamorphosis to the sedentary form. The barnacle benefits by finding a habitat where nutrients are available. (In the case of lodging on the living organism, the barnacle is transported to new sources of food.) The presence of barnacle populations does not appear to hamper or enhance the survival of the animals carrying them."