If diamonddiceclay comes along and says "it's all genetics and diet" don't listen to him.
It's mainly diet, but you have to train your abs to bring them out so they can actually be seen. If it was all diet and genetics, then none of the pros would do abs exercises now, would they? But they all do.
I think the best exercise for abs is hanging knee-raises. You can do this on the thing which puts your back up against a pad, or you can hang from the chin-up bar (although that hurts my shoulders and my hands start to give out after a while). Anyway, most people start with their legs hanging normally, and then bring their knees up.... I think there's a better way to do it, which works your abs more and your psoas, iliacus and other hip flexor muscles less. Starting position should be with your knees up and your thighs parallel to the ground. Then bring your knees to your chest (or as high as you can get it) and then being them down again so your thighs are parallel again. If you can't do anymore, you can do a few reps with the full range of motion.... kinda like forced reps. But try to make the majority of your set with parallel being the starting position, and go as high as you can with your knees.
Sets of 10-20, depending. If you're hanging from the bar, you can twist yourself sideways and do side knee-raises, which work your obliques and serratus some more.
Other exercise for lower abs, is decline board leg raises. Those are good.
For middle and upper abs, I do oblique crunches.... and then standard crunches for upper abs.
That's what I do, anyway.