As the decades have progressed, so have the way music videos are shown to the audience. Typical conventions vary per decade and what was the norm then is completely different to the norm twenty years. A lot of conventions vary between genres as well for example Metal music videos have seen a drastic change over the years.
An early example of a metal music video is 'Rock 'n' Roll all night' by Kiss.
http://www.youtube.com/watch?v=AAt7YbX0T9k~
In this video we can see how everything is big and in your face. The point of the video was showing off the character's fame and basically just showing them on stage to a crowd. The drumkit is big, the movements are big and it's generally over the top. The colour scheme is very dark and the cuts are very fast to reflect the pacing of the music. At this point metal was very self aware, the videos were fun and mirrored the theme of going to a gig, energetic and fun. Recently these videos changed to have a more serious tone, in some cases even shocking.
Slipknot's 'Psychosocial' is a good example of how metal music videos take themselves more seriously nowadays. The dark lighting remains but everything about the video seems threatening in some way or another. There are flames everywhere and most of the actions the characters do are violent in nature. This is coupled with quick camera cuts and effects to make the entire thing seem like something out of a horror film.
I figured it would be a good idea to compare these two videos as they share a lot in common whilst also being very different. They very much represent the period of time they come from. Whilst the Kiss video glorifies everything there is to do with the music the Slipknot video almos has the band blend into the chaos that is around them. Obviously you could draw comparisons of these videos to the oversaturation of the music industry but i felt that would be reading too far into it.
There are three forms of narration seen in music videos nowadays, these are:
-Illustration
-Amplification
-Disjunction
Illustration:

Illustration is the most basic form of narration, typically the video will simply show what is being sung in the lyrics and match the pacing of the music. There is always a strong connection between the video and the music however there is rarely and underlying meaning, it is just illustrating what is being sung.

Amplification:
Amplification is when the video 'Amplifies' the music or lyrics, usually having a close tie with the music but telling its own story in the process. These are usually the trickiest music videos to produce as they take some artistic flare to create, the finished product is usually much more remembered than the other kinds, however.
Disjunction:

Disjunction is when the music video has little to no correlation with the music at all. sometimes this means it tells its own story, but sometimes it is shown as a series of random images just to fit the song. Usually this is done by a director or storyteller sinply telling a story using a different medium than film or literature. These can technically be either the easiest or the hardest music videos to make seeing as they have the most creative freedom of the three. Disjuntioned videos tend to be some of the most well remembers videos however (in some cases this happens even when the song isn't memorable at all.)