A frequently made and innocent enough assumption is that by working out a particular area of your body, you can increase the muscular definition of said area. In this case, people assume that if you do enough ab work, you'll then be able to reveal a set of chiselled abs. Unfortunately, people who think this way are making the error of confusing two completely separate issues; muscle building and fat loss.
Muscle definition (including 6-pack abs) is the result of a combination of adequate muscle mass and low levels of body fat. You can workout your abs as much as you want, but if you have a layer of body fat covering your stomach you’ll simply never be able to see them. To that same accord, you can burn as much fat as you want using cardio or by implementing a calorie deficit, but unless you have adequate muscular development of your abdominals, you’ll likely see no discernible ab muscles to show off – though you can achieve a ‘flat' stomach this way if that’s all you wish to achieve – I wouldn't consider this to be the route to great abs.
What about ab exercises, won’t they burn body fat? While, of course, any movement you perform will use calories, ab exercises are not particularly effective calorie burners when compared to most forms of weight resistance or cardiovascular exercise. It’s also worth mentioning here that working a particular muscle group will not burn the fat directly covering it – that’s known as spot reduction and it's an absolutely falsifiable myth. You can use exercise to burn fat, but this fat will be taken from your entire body in an order and amount that is dictated solely by your genetics. That said, if you’re doing ab exercises in an attempt to simultaneously build abdominal muscle and burn fat over your whole body, you're spinning your wheels.
Your abs are no different from any other type of muscle in that they should be worked out with reasonably heavy resistance for approximately 3 sets of 10-12 repetitions per exercise. If you’re regularly doing compound exercises as part of your normal workout routine (squats especially), then there really isn’t any need to be doing more than 1 or two exercises that specifically target your abs a week. Doing hundreds of crunches or sit-ups will not be more effective at building muscle than the above discussed amount – in fact you’ll likely see less development if you decide to take the latter route.
Not only will you be failing to build muscle in an optimum way, you also won’t be effectively burning body fat by repeatedly performing ab-targeted exercises. To lose fat and thus reveal any muscular mass in your abdominals you need to be in a daily calorie deficit. How you achieve this calorie deficit doesn’t really matter. You can either eat less calories than you need per day or exercise enough to create a calorie deficit, or a combination of the two (what I recommend). To learn about these concepts in more details please give my post on calorie intake a read (opens in a new window).
If revealing a set of firm, flat and defined abs is a goal of yours, then you need to approach this problem holistically. That is, a combination of controlled daily calorie consumption and an ongoing strength training program. To hurry the process up you can use aerobic exercise to create an even greater calorie deficit. This is the only effective way to get the results you desire and it's by far the fastest way to achieve them.