All of the above posts are correct. I just want to add some minor details. For HD channels only, such as channel 904 for HD-NBC rather regular channel 4 for NTSC NBC in my case, prime-time shows broadcast in HD would fill up the plasma screen. When it comes to commercials, those are not in HD. Therefore, they don't fill up the entire screen, and the black bars show up, so whenever the signals are not in HD, such as commercials or non-HD programs, the TV screen won't be fully filled.
Different HDTV signals?
When I tune into HDTV movie channels, the picture fills the entire screen on my 51" widescreen rptv. However, when I put on some of the HD major network channels, like CBS, NBC and Fox and some others, there are bars on the sides of the screen regardless of the picture setting. I'm using cable to receive the signal. Are there two (or more) different HDTV signals being broadcast?