Just doing loads of sit-ups won't work all your abdominal muscles - you need to work upper abs, obliques and lower abs to get a six-pack. It is important to follow an exercise plan that concentrates on the entire abdominal region. Working the muscles in your torso, hips and abdomen will give you improved balance and stability, and also help protect you against back problems.
A six or eight-pack is actually only one single muscle - the Rectus Abdominis. The muscle is divided into a grid by tendons - three horizontal and one vertical.