Most HDTVs have a Format button, which lets you cycle through "Normal", "Zoom", and "Stretch" modes.
Normal means the picture is displayed the way it's being transmitted. For non-HD material, this means you'll see the picture in the middle of your screen, with black bars on the sides. For HD or widescreen material, the picture should fill up the screen.
Zoom keeps the aspect ratio the same, but "zooms" in on a section of the picture so that it fills your picture. The result is when watching a non-HD TV show, the picture fills up the screen, but sometimes the tops of people's heads may be cut off. However, people will appear normal.
Stretch, meanwhile, alters the aspect ratio. It takes the full non-HD picture, but expands, or stretches, it to fill the entire screen. The result is that people appear slightly fatter than they normally would.
Try changing the format mode on your TV to see which mode you prefer. I personally like Normal, even though I end up with black bars on the sides of my screen.