What is symbiosis, and what types exist?

When two or more species live in direct and intimate contact with each other, their relationship is called symbiotic.

Mutualism is a symbiotic relationship where both organisms benefit (+/+).

Commensalism is a symbiotic relationship where one organism benefits and one is unaware of the organism (+/0).

Parasitism is a symbiotic relationship where one organism, the parasite, benefits while the host is harmed (+/-).