Sit ups, you're actually sitting up. Crunches, you're bring your chin closer to your chest without moving your abdomen or any part of your lower body.
two hundred sit-ups
Even though its called situps, they're actually advocating crunches. There's alot of evidence that situps can lead to painful back injuries and so many places recommend you don't do them anymore as there are other excercises to work your hip flexors.
I think it's because people over work one area without working on the counter balancing muscle, or so I've been told. And it's a lot of pressure on your lower back.
Unfortunately you can't spot reduce (make a part of your body specifically smaller) - crunches etc. will help you a lot but only if there's not a layer of fat covering it and where it comes off is sadly determined by genetics - maybe your tummy is just the last to go?