Writers are said to be either plotters or pantsers. Meaning either that they plan out the chapters in their stories ahead of writing, or they write by the seat of their pants—with little or no advance planning.
If you are a plotter, knowing about using a plot outline is essential. A plot outline is a thorough summary including the basic plot twists in each chapter. It is written before you write your book and is used to guide your writing to completion. It is much longer than your synopsis will be.
Both plotters and pantsers will need to write a synopsis which summarizes without giving away all of the details of the plot. It is written after you finish writing your manuscript. In it, be sure to include what is unique about your story and/or characters.
Remember, the synopsis will be used to pitch your story to agents and publishers. Use your plot outline to get you to a satisfying end.