For Standard Definition (SD) TVs choose Standard (4:3) only. You'll then be asked to choose how you want HD content to display on your TV, either by letterboxing (putting black pillars on the top and bottom of the content) or cropping the content to fit.
For Widescreen TVs, determine how standard content displays on your TV by first choosing Widescreen (16:9) and then selecting maintain (displays standard content as is) or stretch (stretches the standard content to fill the TV screen).
