Here is a primitive diagram of the Hero’s Journey. Basically, it follows Aristotle’s story design of Beginning, Middle, and End.
For writers, this structure also mirrors the 3-Act play, where both the “Descent” and the “Initiation” make up the middle act.
The Ordinary World isn’t necessarily “ordinary”. What is meant by the term “ordinary” is the day-to-day world the Hero finds him/her-self in. The “ordinary” world can, in fact, be quite fantastic. The “ordinary world” of Stars War (Star Wars, Episode IV: A New Hope ) with interstellar travel is not “ordinary” in our typical understanding of the word. Do not be misled by this nomenclature.
The Ordinary World is in contrast to the World of the Adventure or Quest otherwise known as the Special World. The extent of the contrast is determined by the needs of the story and skill of the writer.
Typically, in the Ordinary World, the community is experiencing a threat or, in some manner, life has become out of balance. The Ordinary World is the launch pad: commonly this is where the Hero is first introduced in the story. Frequently the Ordinary World is static but unstable. The situation may be known, but is now escalating or becoming radically unstable.
The Hero’s Journey is a quest to restore balance to the village; hence the Departure or Separation.
Then come the Trials and Tribulations of the Special World. The boon is not easily accessed or readily offered. The Hero has to work for it in the Descent and Initiation.
However, the story does not end at this point. What is the benefit if he/she cannot restore balance to the village? Therefore, the Hero must Return. There are forces at work here that challenge the Hero’s Return. The Hero leaves the Special World and returns to the Ordinary World to bring the boon back to the community that he/she has gained. The story is in a crescendo to the climax in the Return with a brief denouement displaying, or at least implying, that order has been restored.
For detailed discussion on The Hero’s Journey, mythology and writing see The Hero with a Thousand Faces and The Writers Journey: Mythic Structure for Writers. For examples of the Hero’s Journey in film, see Stuart Voytilla’s Myth & the Movies.