At its core, the Monomyth is a basic plot structure which can be found in many mythic tales of nearly all cultures. It was first described by Joseph Campbell, an American professor who gained renown primarily for his work in the fields of comparative mythology and religion, in his book The Hero with a Thousand Faces. Also known under the terms "Hero's Journey" or "Heroic Quest", the Monomyth contains three basic parts:
Before I come to further, more detailed subdivisions of these three basic parts, it is important to note the criticism Joseph Campbell's Monomyth-thesis has encountered over the years – criticism that stems largely from the fact that he went as far as claiming that this structure is inherent to all mythic and, basically, even other stories, as many of them require such a level of abstraction to still fit this pattern that any connection seems more constructed than real. In addition, Campbell's strong reliance on psychoanalysis to explain why the Monomyth can supposedly be found in all cultures – he writes that the different stations of the Hero's Journey symbolically represent collectively subconscious archetypes of different stages of human development in keeping with the Freudian model – has also encountered stiff headwind.
I would argue that both points of criticism are sound, but that some scholars' condemnation of Campbell's entire work as "New Age garbage" is as much an invalid generalization as those Campbell makes. While his desire to construct a huge, all-explaining 'world formula' for stories as well as the reliance on a branch of psychology that was widely considered outdated even at the time he wrote his book are problematic, The Hero with a Thousand Faces still offers one of the most comprehensive descriptions of stereotypical plot patterns that are, if not universal, at least recurring quite often in many mythic stories of very different cultures all over the world. In this context, it seems a reasonable enough point of entry for a closer examination of the Star Wars saga, especially since its creator George Lucas used Campbell's book as a kind of guideline after which he modeled his tale.