Arc is a literary term that conceptually makes sense but is difficult to apply in story planning – that is, if you’re the type of writer you can’t figure out endings.
Arc is the storyline from beginning to middle to end.
In my novel “Dropping Colors,” I’m a third of the way into the story – I know that my two main characters, an artist and a musician, are going to meet, engage in some kind of romance and change each other’s realities. How my story will end is up to my subconscious and the process of letting the story unfold.
The arc is a very loose description of story structure, similar to how the architecture of a home can be reduced to the walls, windows and doors.
The structure of a story contains the elements of the arc line but with more detail. Stories need to have an origination, or some kind of incident that sets up the conflict. This is the beginning.
The middle is the escalation of that conflict and a complication of the situation the characters have to face. The ending resolves the conflict and situation, offering a resolution, unless the story is part of a series.
The storyline, in that case, is resolved but something brought up in the telling sets up a new conflict that can be continued in the next installment. Or, in the case of mysteries, one case is closed but there will be another as part of the character’s job or hobby.
Alongside the story arc, there is character arc.
The character arc is the line of the character’s transformation from the beginning to the middle and to the end. The line shows how the character faces her flaws, fears and limitations and overcomes what hinders her from getting what she wants.
The arc, in other words, is the personal growth and development that she undergoes in a story.
See Zoey the dachshund’s take on arc at http://zoeyspaw.wordpress.com/2012/05/27/a-dogs-story-journey/#comments