Editing entails the assembly of individual shots into complete scenes, and individual scenes into complete movies. The editor starts with dailies--the raw footage shot on set--and selects the best takes of individual shots. Those shots can then be spliced together into a coherent form, creating the ebb and flow of the scene. Editors used to assemble their cuts from strips of film. Today, they use computer programs to make the process much easier.
A film's music is important for establishing the tone of the piece: enhancing suspense in horror movies or accentuating the sentiment or romance. Since music needs to be cued to the action of the scene, it can't be formally added until post-production, when the composer can adjust its length to fit.
Members of the production staff called foley artists create a separate soundtrack covering the various sound effects in a film. This can include things like windows breaking, cars slamming on the brakes and even specific effects like R2-D2's beeps. In addition, the principal actors may be called in to re-loop dialogue to provide a clearer or more emotionally truthful rendition of the dialogue that matches with their lip movements in the picture.
The various soundtrack elements--including dialogue, music and sound effects--need to be blended into a single tapestry of sound. A mixer blends them together, deciding which element is important during which moment and taking care to sync the sound up with the image. Otherwise, the music might drown out a given piece of dialogue, or a comparatively quiet effect like a door closing may sound like a cacophonous roar.
Visual effects run the gamut of scenery created from scratch to laser bolts from ray guns to entire characters created using motion-capture technology. Such images are usually composed by computer. The scenes are shot with the actors on green screen; then the background images, sets and the like are "painted" in by a 3-D effects artist. In some cases, for example, the White Rabbit in Tim Burton's "Alice in Wonderland" or Gollum in "The Lord of the Rings" trilogy--an actor will perform in a special suit that records his movements. Effects artists then can use that as a framework to construct a character. All of that needs to be added after principal photography is complete.