Whenever two organisms of different species exist in close physical contact to the benefit of both organisms, that's symbiosis. Symbiosis can occur between animals, plants, fungi or any combination thereof. Each organism contributes something that benefits the survival of the other, and in turn receives a survival benefit of its own.
Some symbiotes are so closely intertwined that it's difficult to tell where one organism ends and the other begins. And in the case of plant/animal symbiotes, it can be difficult to tell whether the organisms are plants, animals, or a little bit of both.
Symbiotes aren't cartoon animals living and working together in perfect harmony. Most symbiotes have no idea that they're helping another creature. They're just surviving in whatever way works best for them, an instinctive behavior driven by natural selection.
You probably didn't realize that you're a symbiote yourself. Or that life on Earth probably wouldn't exist without symbiosis. Or that symbiosis might have been responsible for the evolution of multicellular life. Or that some scientists think the entire planet is one giant symbiotic organism. Sometimes symbiosis is pretty weird. We'll explore the weirdness (and maybe find out that it's not so weird after all) in this article.