A music video is a short film or video that accompanies a piece of music. Modern music videos are primarily made and used as a marketing device intended to promote the sale of music recordings.
Then can be as simple as the band just playing thier song live at a show that was recorded such as Mumford & Son's Roll away your stone (YTB) to a ellaborate video that involves animation or big set pieces such as Freak on a leash (YTB) by Korn.
The videos can be very cheap for instance The Pixies's video for Velouria (YTB which didn't cost anything because the band just filmed themselves running over some rocks for 24 seconds or as expensive Michael Jackson & Janet Jackson's Scream which had a budget of 7million.
It would be hard to characterise music videos as a whole because every music genre has its own codes and conventions, but they is a way you can breakdown music video.
They are three different kinds of music videos.
- Narrative videos - These videos consist of imagery usually relating to the narrative of the video. Some examples could be Wheatus - Teenage Dirtbag (YTB) and Fountains of Wayne - Stacy's Mom (YTB)
- Concept Videos -Concept videos are basically a video that is just a concept, usually completely random and is no way related to the lyrics. The concept could be a story or just a animation. Examples being OK Go - This Too Shall Pass (YTB) and The Island part 1 (YTb) by Pendulum.
It is usually the case where these three types of music videos overlap, For instance a band playing or singing in the background of a concept video or a concept during a narrative video. Nearly all the examples i gave are mixing different music video types. Sometimes all three can be seen in one video for instance in the video below.