16:9 is just the aspect ratio of the tv (ie the dimensions of the picture). HD vs. Standard actually refers to how many lines the tv draws across your screen to create the image. To be HD, it has to draw at least 720 lines per image (1080 is the other common HD standard), while "normal" American, non-HD televisions only 480 lines per image. In theory, you could make an HD television that's in 4:3, or a non-HD tv that was 16:9, but for some reason they don't. I don't know enough about the history of HD to know why the aspect ratio standard changed with increasing the definition.