Symbiosis is a close and long-term relationship between two organisms. Symbiosis can be helpful or harmful to either one or both partners within the relationship. Mutualism is a form of symbiosis in which both organisms benefit. Commensalism is a form of symbiosis in which one organism benefits, but the other is neither harmed nor helped. Parasitism is a form of symbiosis in which one organism is helped and the other partner within the relationship is harmed.
An example of mutualism is the oxpecker and the zebra. An oxpecker is a bird that resides on the back of a zebra. The oxpecker eats the food off of the zebra while the zebra is cleaned of parasites.
Another example of a symbiotic relationship (parasitism in particular) would be a tick living under a dog's skin. The tick benefits from using this environment as its home, but the dog gains no benefit.