Okay, it's been awhile since I read Campbell, but I'll give it a go.
Joseph Campbell's point, as I see it, is not that there is no such thing as an original story, (which seems to be the most common Campbell-related misunderstanding) but rather that all stories that evoke a certain reaction from their audience do so because they encompass certain themes common to the psyche of all humans. He theorizes that Mythos is as essential to human mental/spiritual development as DNA is to the physical. In every human life, according to Campbell, there are certain points at which we come to a crossroads, a spiritual threshold we must cross, and grow - or refuse to cross, and remain spiritually immature. These thresholds occur most commonly at points in life where great physical change is also occurring, although there are others more intuitive. The obvious ones include birth, the onset and the decline of puberty, marriage, old age, death. In ancient societies, (and a very few even today) these thresholds were often approached with a public acknowledgement of some kind; eg. Native American Vision Quests, or ritual scarring - the crucial Rite of Passage..
So, Annie, what does that have to do with anything we're discussing here, you ask?
Well, (I really am trying to keep this short...
) Campbell asserts that stories, specifically adventure stories, serve in essence the same cathartic purpose that a more physical Rite of Passage does. When we read (or hear) a true adventure story, we experience vicariously through the hero (and, as defined by Campbell, TC fits this description perhaps more acutely than many other protagonists....the term "anti-hero" means little in the context of the mythos) the transformations that are so necessary to our own psyches.
Furthermore, Campbell maps out in detail the specific requirements of the hero's journey, in order to achieve this desired catharsis. He names, as Tulizar has said, twelve points along the road to adventure that the Hero must typically reach, and pass through, in order to complete his journey. These points in the story are common to many tales from many different cultures and eras around the world, this fact being the basis for Campbell's theory that all people share, at the most primitive level, the same psychological needs. These needs generate myths. The needs are the same, the myths are fundamentally the same, and Cambell has coined the term "Monomyth" to fit this phenomenon. It's The Myth, the one constant, unending story of Mankind.
I'm not upholding Campbell as the definitive authority on Mythology and the Human Consciousness, but this is a subject I love, and the man has some very interesting ideas. I could attempt to go on about it - but there's much more to it that, unfortunately, is none too fresh in my mind right now. This is the best I could do with my limited recall at the moment,
but if this type of material interests you, I would highly recommend Campbell's Hero With a Thousand Faces, or The Power of Myth as very worth your while. It's fascinating stuff.
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