The traditional definition of symbiosis is a mutually beneficial relationship involving close physical contact between two organisms that aren't the same species. Most biologists still adhere to this definition. Some biologists, however, consider any interspecies relationship involving frequent close contact to be symbiosis, regardless of which of the organisms benefits. This includes commensalism, in which one organism benefits and the other isn't affected much at all, and parasitism, in which one organism benefits and the other is harmed. In this article we're going to focus on mutually beneficial symbiosis.
There are several forms of symbiosis. In some instances, the organisms require the symbiotic relationship in order to survive. This is known as obligate symbiosis. In other cases, the symbiotic relationship gives each organism a greater chance of survival but isn't absolutely necessary. This is known as facultative symbiosis. Symbiotic relationships aren't always symmetrical -- they can be obligate for one organism and facultative for the other.
The "close physical contact" part of the definition is worth looking at more closely. In most cases, it's fairly straightforward -- one organism may make its home directly on another organism's body, or even live inside it. But biologists also consider the biochemical relationship between two organisms. If they're generating and sharing enzymes, proteins, gases or other chemicals then they can also said to be symbiotes.
Endosymbiotes live inside another organism. And by inside, biologists really mean inside -- in between cells or within the body tissues (like the acoel flatworm). Ectosymbiotes live on the body of another organism. (Note that organisms that live within another's digestive tract are considered ectosymbiotes. Apparently living in someone's else's intestines doesn't qualify as a close enough relationship for biologists to call them endosymbiotes.)