MUSICAL
AKA Beat synch. Musical synchronization is the simplest, conceptually speaking. This is the idea that cuts, fades, effects, or just events in the video are timed to match the audio. While sometimes simple to execute, this can get very complicated very rapidly. Many songs will have more than one musical line going at a time, and sounds of varying volume and tone. As an editor, you have to pick what you think is important, and synch to that. Sometimes the music itself presents a clear lead line to work with, but this is frequently not the case. What you show, how strongly you show it, and how you show it are all choices to be made.
For example, A strong guitar riff might be matched with a hard cut, or a snare drum hit with a lens flare. A bass drum hit might be paired with an explosion, particularly if there are a series of bass drum hits in an action video. Long, slow, lilting musical passages may be coupled with equally long and slow crossfades. This can be layered, with certain characters, visual effects, or types effects bound to specific sounds. Perhaps a series of explosions timed to a particularly hard set of guitar riffs.
Most videos make use of beat synch heavily. To ignore the music itself is generally considered extremely bad form, although there are exceptions. Without extremely good reason, such as comedy, the video ending, etc., missing a significant change in the music is considered a major slip-up on the part of the editor.
Example of Musical Synch: Jasper-Isis and krzT321's "Absolution"