Film editing is an art of storytelling practiced by connecting of one or more shots to form a sequence, and the subsequent connecting of sequences to form an entire movie. Film editing is the only art that is unique to cinema and which defines and separates filmmaking from almost all other art forms (such as: photography, theater, dance, writing, and directing). It is often referred to as the "Invisible Art", since when it is well-practiced, the viewer becomes so engaged that he is not even aware of the work of the editor.
The job of an editor isn’t merely to mechanically put pieces of a film together, nor to just cut off the film slates, nor merely to edit dialogue scenes. Film editing is an art form which can either make or break a film.[1] A film editor works with the layers of images, the story, the music, the rhythm, the pace, shapes the actors' performances, "re-directing" and often re-writing the film during the editing process, honing the infinite possibilities of the juxtaposition of small snippets of film into a creative, coherent, cohesive whole.
Film editing is an art that can be used in diverse ways. It can create sensually provocative montages. It can be a laboratory for experimental cinema. It can bring out the emotional truth in an actor's performance. It can create a point of view on otherwise obtuse events. It can guide the telling and pace of a story. It can create the illusion of danger where there is none, surprise when we least expect it, and a vital subconscious emotional connection to viewer.
from: wikipedia.org