Editing can mean lots of things, from basically shortening a clip to adding a massive explosion. it all starts with one thing, raw material.
a simple video of a villain standing infant a geen/blue screen can get transformed to a video of the villain standing in his evil layout. while the hero stands there and just before he explode he get cut away and replaced with a digital version of himself.

video clips:
Flashing effects quick cuts and cool transformation, while making sure it goes on the beat of the music.

Visual Effects:
Transforming green/blue screen video's to a whole different location/world.
Adding thing in that weren't there before, such as explosions or blood. Anything you can think off

Montage:
How long wil this scene will go on for? what kind of transition wil these scene's have? That's all montage
its similar to a puzzle, you have all the pieces but the image is not complete yet. All the scene's have been edited and made perfect but now its up to the montage to make everything flow together to make a good story with nice transitions.