Stomach crunches are similar to sit-ups but easier. They're easier on your back too. Actually, sit ups really aren't that effective if you want to exercise your abs. Sit ups work your psoas muscle, which is more responsible for your back than your stomach. Stomach crunches are much more effective for working your abs.
Technically, "stomach crunch" is not the correct term. "Ab crunch", or even "abdominal crunch", is a more precise term, as the crunch affects your abdominal muscles - not your stomach!
Stomach crunches are best performed the end of your workout. The movement should be slow and controlled. Don't be tempted to rock yourself back and forward to try to get a few more out. It's better to do fewer crunches with good form than to do more crunches with bad form.
Performing the Stomach Crunch
The stomach crunch works your lower stomach (or should I say, "lower abs").
Lie on the floor with your legs bent, your feet flat on the floor. If you prefer, you can keep your legs bent at a 90 degree angle so that the bottom of your feet are vertical.
Place your hands on the SIDE of your head, or if you prefer, cross them over your chest.
Lift your shoulder blades off the ground moving your elbows towards your knees. Don't try to lift your whole back - just your shoulder blades.
Squeeze your abs for a second.
Slowly return to the starting position.
Repeat steps 3 - 5 until you've completed the desired number of reps.