Timeline terminology
Introduction or Prelude:
The introduction, or prelude would be those very first notes in the song, before the lyrics begin; often these notes make the song instantly recognizable, the part where you go "YEAH! Turn it up!" when you hear them on the radio. Older rock songs made a point using short preludes, but they seem less common with more modern groups. Think of the classic rock song “Start Me Up” by the Rolling Stones, or the guitar stings that start off most ACDC songs.
If you listen to live music, you will notice that as soon as a band starts to play the distinctive notes to a popular song that is when the crowd begins to cheer, obviously they've recognized the intro.
The Bridge:
The bridge is a short section of music that connects one part of the song to the next. Not all songs have an obvious bridges, but there is almost always a key note, or series of notes. My theory is that this lets the rest of the band know when it’s time to start playing new stuff for the song in concert. I could be wrong. Typically bridges occur between major points in the lyrics, so you can always use the transition from one chorus of lyrics to the next.
Use the bridge is used for you to make your transitions from beginning, to middle, to end, and possibly a coda.
The beginning:
The beginning is when you set the stage and tone of your video. You need to spend time building your video; it’s environment, characters and whatnot. This structure is very important in order to give an appropriate flavor to your video. Just like many people have a difficult time appreciating non-functional art, or Phillip Glass, many people will have a difficult time with your video if you skip the structure. If you video does not have a beginning, and just jumps right into whatever main theme you have chosen, then you risk confusing the viewer. Don’t confuse your viewer accidentally.
Middle:
Consider the middle as the meat of your video. For most songs the middle portion is the longest, and this is when the story of your song, and video are given a chance to play out. Some songs will actually have another bridge to extend this 'middle' section of the video, so be aware of that. You can think of it as two 'middles' if you like, and perhaps use it as a chance to focus on two characters for example.
End or Finale:
This is the end, my friend...exactly like what it sounds like. This should be when everyone dies, gets the girl, wins, blows up the enemy, or whatever. This will also mostly likely be the loudest, most dramatic part of your song as well. The finale may not necessarily come at the end of your song.
Not to be confused with the climax, which is the part that peaks musically, this is the end segment.
The climax should be in this segment, although for some songs it won't be. If the 'climax' is in this segment then use it like it was meant to be, if it occurred earlier in the song (examples would include stairway to heaven - Led Zeppelin, and King of pain- Sting) where there is a climax, nowhere near the end of the song) then be creative and use the end to wind down your video.
Epilogue:
Every once in a while you will have a song that very definitely ends, yet continues to play on. Usually these songs are just repeating lyrics, or some section of the song over and over until it fades into silence. If you have one of these, then this is the time to basically "compile" your story. I recommend flashbacks, or similar devices for songs with this structure. You can also use these to begin fading into your end credits if you so desire.