Stories have three acts. Some of you might disagree with me. Good! I don’t want you to agree with me all the time. I want you to enjoy my opinions, and interpret them in whichever way you see fit.
There’s always a beginning, a middle, and an end. In some stories this distinction can be observed more prominently than in others, but every story consists of three parts. Unless you write like William Shakespeare. Even this is just an interpretation of the three-act structure, where the middle part is stretched over acts 2-4. This is a topic for another post, though.
A beginning has the obvious function of introducing your readers to your characters who they will accompany on their adventures, in the world you created for them. Whether you choose to bring in the reader at a very early point in a character’s life, or come into the story much later, it’s no doubt an important part of any story. It’s already here your reader will find important clues as to how the characters might develop and grow over the course of your novel.
The middle part consists often of tests, or thresholds if you prefer that term, your characters will endure to make them stronger, before fulfilling their aim. These don’t have to be negative occurrences always, but of course some conflict should exist to drive the story forward. It can also be a new mystery, or the revelation of one. Just when your readers thought they had figured it all out, surprise them with a twist. I personally don’t want to overdo it though; it must always feel like a natural progression of the story.
As the ending nears, one of two things should happen. Either all the questions your readers had, were answered and all the mysteries revealed, or at least the character's destiny has been resolved. It's hard to appease all wants and expectations. I believe that it's also foolish to want to please everyone. A successful story isn't one that answers all questions necessarily. A successful story is one that has integrity and coherence within the parameters of its own structure and world.
I boldly state that every story has three acts, and yet it may not always be apparent. Look closely though, and you'll recognise three acts in most any novel you read. It may indeed be easier to observe in epic tales, in which a hero or heroine embarks on a journey that will test their abilities. In such cases it'll always be possible to identify the beginning, the middle, and the end. To start with, there's a call to adventure (Luke Skywalker meets Obi-Wan Kenobi). Then comes the first, of perhaps many, thresholds (Luke's foster parents are murdered). Finally, the protagonist will come to the end of an arduous adventure (Luke destroys the Death Star). With this standard plot map in hand, you can deconstruct any story, even those which are non-linear.
While I strongly believe every story falls snugly into three acts, you might understand each act and its function differently. Here's where the beauty of variety comes in. We're all unique in how we use language and structure to shape the stories we want to tell. What I think is a good ending might be a horrible ending in your opinion. A writer's existence is highly dependent on an incessant learning process, and we never stop finding new ways to express ourselves.
P.S.: I'm a huge Star Wars fan. This explains the reference to Episode IV.