Strengthening your core strengthens your abdominal muscles and provides greater overall stability. All of your movement starts in your core.
If you want to devise an abdominal exercise program, include forward flexion exercises, side flexion exercises and rotational exercises.
Crunches are a type of forward flexion exercise. Side bends are side flexion exercises. Standing twists are rotational exercises.
"The transverse abdominals are connected to your back muscles as well as your rectus abdominals. The rectus abdominals are your vertical abdominal muscles and are the muscles most commonly targeted when you do stomach exercises."
Crunches target the rectus abdominals. Bicycle exercise also focuses on the rectus abdominals and the obliques. The Plank works abs and back muscles along with the stabilizer muscles. Leg Scissors can quickly bring muscle tone to the abs. The Long Arm Crunch brings a little extra challenge to traditional crunches, focusing on the upper rectus abdominal muscles.
http://fitho.in/2010/07/27/ab-toning-exercises