Summary of editing:

The filmmaking is involved with the combination and selection of the shots into sequences. This is a unique art in the cinema where the editor make the effect for the viewer, that is so engaged to the story that doesn’t realizes about what has been changed. This is called the "invisible art".
The film editor must know how to tell a story, be politically savvy when working with directors and studio executives, and have a calm and confident demeanor. Millions of dollars of film and the responsibility of guiding the picture through post-production and into theaters rest in the editor's hands. Scenes may have been photographed poorly and performances might have been less than inspired, but a skilled and creative editor can assemble the film so that the audience will never see these imperfections.
The film editor, working with the director, shaped the scene into its final form. After hours and hours of reviewing the unedited film, he created this one-minute scene.
Cuts and Transitions: Assembling the Scene
Editors select sounds and images from all the film that has been shot and arrange them to make the movie. They also plan how one shot will best transition to the next. There are dozens of possible transitions the editor can choose, each of which will create a different feeling.

Editing often begins as soon as film has been shot. Early scenes are assembled for the producer and director to view. Occasionally, the actors will also view these early scenes.

The first cut of a film, called a "rough cut," takes up to three months to complete. The Sometimes the editor works alone, sometimes with the director. The sound designer and music composer join them for the final cut.
Today most editors use computers or nonlinear digital editing systems to compile a film. The editor arranges the work print, and then creates an edit decision list that will be passed on to the negative matcher.