Elvira Sweeney is a Vidder. A vidder is someone who makes fan and fan tribute videos. They also at times create new videos based upon aspects of the original source.
Vidders, like videobloggers and those working in web video are having a dickens of a time with the ever evolving problems of maintaining the desired video frame size and aspect ratio. Many of the free web hosts are displaying videos in the 16:9 format.
Not all of us have HD camcorders. There are a lot of standard definition 640x480 camcorders out there, and standard definition wide screen camcorders as well.
In this video, Elvira does a good job of demonstrating the different aspect ratios of 4:3, 16:9 why it is important to pay attention to how you create and export your video to keep it in the proper ratio.
Just a few things, when she says DVD you think camcorder video format size. Also, I don't have a problem with the letterbox 16:9 bars but Elvira doesn't like them. But I do agree that if the video was shot in 16:9 you shouldn't see bars or pillars on the side.
For more information you can go to Elvira's Aspect Ratio page where she describes how to fix the problems of squishy videos.