The other two posters are absolutely right in a number of things that they state about developing a six-pack, including that it usually takes a lot of work and dedication and that it's something that comes more easily to some people that others.
What they hint at but don't mention explicitly are two things that I think are pretty important:
1. Lowering body fat while still building muscle is essential is you want the six-pack to be visible. You can build the muscles even when they're hidden under layers of fat. You need to watch your diet, burn fat through cardio (e.g. running), and read up on other strategies that studies seem to show help as well (e.g. drinking a full, tall glass of cold water every morning before you do anything else).
2. Do the right exercises. Some of the best exercises for the abdominals are not the standard, lie-flat-on-your-back-on-the-floor sit ups. (That's a good place to start, though, if you're just starting to work out.) Hanging leg lifts are much more effective at challenging the abdominal muscles. I've attached two YouTube videos showing how they're done.
I'll close by encouraging you to think about your goals, to work with your body, and to find ways to make the regimen enjoyable. Focus not just on appearance but on coordination, strength, and endurance, too. You'll be much better off in the long run.
If the question is related to strengthening one's abdominal region, the answer lies in a strict workout regimen. I would suggest doing abdominal crunches, similar to sit ups. I think that crunches help out a great deal as little is needed to do it. Lying flat on the back and then keeping the body straight and moving about 90 degrees up, or a little else, while breathing as you move up will help to strengthen the core area. This target the abs. In addition to this, I think being able to space out your ab workouts, such as four sets in the morning and four sets in the evening will help out immensely. If you could work in some running in addition to this, I think that this would work very well in terms of gaining the elusive "six pack."
I agree with the previous post. I have taken lots of weight training courses at college and work out regularly. It is important to do cross over when doing crunches as well. They help to contract the muscles at a different angle and will help to define the abs even better.
Some people are better able to sculpt their bodies than others. I have known people who have worked hard to build their abs but never attained a six pack. On the other hand, some of the boys that I raised have never done much in the way of exercise and have great six packs. Body type makes a difference as to how cut you can become. However, don't give up. One never knows what one can do until one tries.