The Hero with a Thousand Faces written by Joseph Campbell describes an archetypal pattern that he discovered which underlies all stories across all cultures and time periods. The most famous application of this theory is, of course, Star Wars.
Hollywood has been in a continual quest to find ways to make creating good stories easier. At one time there were two story paradigms in Hollywood. One was a fish out of water and the other was an unholy alliance. Beverly Hills Cop would be an example of a fish out of water and 48 Hours would be an example of an unholy alliance. From what I heard, if your script didn't fit either of these moulds then it would be deep-sixed.
It was Chris Volger who brought the hero's journey to the attention of Hollywood with a memo he wrote. I was fortunate to take a class at UCLA with Chris right before he published the Writer's Journey. I think that his key insight was that the writer, him or herself actually goes on the hero's journey when they write a story. I experientially believe this to be true.
I recommend his book. The Writers Journey
Now, I believe that any story structure theory should be easy to apply and easy to remember. So I am going to go off my memory of the hero's journey trusting that I remembered the important points. The hero's journey is setup like a counter clockwise journey. It maps time around a circle. Why it's counter clockwise I have no idea.
The hero starts in their ordinary world and is awakened to a call to adventure. They refuse the call only to go on the journey after all. If they didn't go there wouldn't be any story, would there? Next they meet a mentor.
The next step is they have to cross a threshold into an underworld. So they have to deal with the threshold guardians. From the beginning until they cross the threshold is ACT 1. The underworld is ACT 2. The return is ACT 3.
So far, so good. This is helpful. Let's look at the rest of the journey with the help of some visual aids.