If your TV's picture doesn't quite look right, you're likely watching standard-definition programming on an HDTV.
HD programming shows at a 16:9 aspect ratio, and standard-definition programming is displayed at a 4:3 aspect ratio. Your HDTV will make up for this difference by either reshaping the image to fill the HD screen causing the image to look stretched or allowing it to display at standard-definition resolution causing it to not fill the whole screen.
Most modern TVs can automatically adjust to the programming it is receiving. To make sure your TV is adjusting automatically, make sure your picture settings are on Auto-Adjust or Normal.