Ernest Lehman, the six-time Oscar nominated screenwriter, is quite succinct in his broad stroke framework of screenplay structure: "In the first act, it's who are the people and what is the situation of this whole story. The second act is the progression of that situation to a high point of conflict and great problems. And the third act is how the conflicts and problems are resolved."
Although correct in his concise breakdown, Lehman knew there was a lot more to screenwriting than just introducing some characters, putting them in a situation, and creating obstacles for them. The reality is a design much more intricate than that, and therefore, the serious screenwriter employs many practical strategies and time-tested structural models to develop a detailed blueprint before writing FADE IN.
It's common sense really. You don't build before you plan, and you certainly don't paint before you prime. The detail is in the preparation, and for the screenwriter, the prep work is what we call The Outline.
Of course, the most detailed outline is no good if you don't first develop a character with a believable want. Any good screenplay is anchored in this: an interesting character, who wants something badly, and is having trouble getting it: (Character + Want) + Conflict = Story.
But even with this equation as the root of the story, there's much more to the final structural design: acts, sequences, plot points, rising action, planting and payoff... and so it goes. Understanding how to execute and flesh out all the specific parts of your three acts and eight sequences while pin pointing your five major plot points is truly the Gorilla Glue of screenwriting.