16:9 is widescreen format and 4:3 is non wide screen.
16:9 will generally show as a letterbox format on a standard 4:3 tv screen, meaning you see the full width that you would on a widescreen tv, but the depth is smaller.
Widescreen DVDs show things more like they were on the original theatrical release, and you will see more to the left and right of the picture that might be cut off on a 4:3 ratio DVD.
