When you do sit-ups, you are using your abdominal muscles. Lots of people do sit-ups, and think they are exercising or losing weight, especiallu when they feel sore in their stomache. They don't realize that they are doing sit-ups wrong, and could be hurting themselves, instead of helping.
If you do sit-ups with your legs extended, rather than with your knees bent, you will be strenthening your hip flexor muscles more than your stomache muscles (most bacl pain suffers need to stretch their hip flexors not streghten them.) Lying on your back with your legs extended will also cause a natural curve in your lower back to become exaggerated and this will put extra strain on your back.
If you do full sit-ups, rather then half sit-ups, you will also e working your hip flexor muscles. After your head and shoulders leave the floor--that is, after about the first 30 degrees--your stomache muscles stop working and your hip flexors kick in.
Since sit-ups don't seem to be very safe, I'm sure your wondering what should you do, well its quite simple, crunches are always the best way to go, half up and half down. When doing a crunchy, make sure your knees are leveled together, and your hands are in a steady postition, sometimes having your hands behind your head is not always the best way top go.