What exactly should I be putting in my Demo Reel?

Hey,


I'm wanting to make a demo reel of some of the stuff I've done so far with video editing, but I'm not sure what I should exactly be putting in it. What do employers look for in demo reels? How well the cuts are made, or what? I'm not too sure because most demo reels I've seen include some type of song throughout it with various clips of their work with normal cuts. Should I keep it simple like that? Or should I try to splice it up by adding some effects in it? Do the employers judge on the quality of the footage in your demo reel, or how well the demo reel is synced? Or is it just showing all the other things you've handled in editing, and they judge on that? I thought it would be hard to judge without hearing the original audio and seeing the hole segment of the clip.