USEFUL INFO
Outline, treatment, screenplay
They are four different documents and they are written in this order. Each one answers a question the previous one left open.
The story outline
It tells the story from beginning to end, without dialogue. Who the protagonist is (if there is one), what happens, how it ends.
The outline is an excellent exercise in condensing what you have in your head into a few pages. If the story doesn't work, the outline shows it straight away.
The treatment
The same story, but developed further.
It is the document you discuss before writing the screenplay, and the moment when a structural problem costs one line instead of forty pages to redo.
The step outline
A list of the scenes, with the heading and what happens. It looks like the dullest thing imaginable and is in fact the most useful tool there is for spotting an imbalance: from the step outline you see at a glance whether one act is twice as long as another, or whether there are six scenes in a row in the same room.
Nobody says you have to write your step outline in order. If you are stuck on a scene, skip it and come back later.
The screenplay
The film, written down. One page usually equals one minute, and it is a calculation producers really make: it works as a unit of measurement.
It is the last document in the chain and the first one someone will read in full.