The rectus abdominus in the front, which give you the "xylophone" effect
The serratus on the upper sides, which connect the abdomen and the pecs.
The transverse abdominus
The obliques on the lower sides, the biggest of the abdominal muscles.
Everybody tries crunches and sit-ups for their abs, but they are almost impossible to do effectively. I suggest the plank (reverse push-up) and side twists.
And cardio: since abs are a matter of definition rather than bulk, you need to get your body fat down.
The definition is most noticeable when the abs are hairless.
A thin line of hair going down the abdominal ridge is called a "glory trail," since it draws the eye to the crotch. Charlie McDermott made the glory trail famous by displaying his in nearly every episode of The Middle.
More after the break.
Or a teenager.
As you age, it becomes harder and harder to maintain the low body fat necessary for defined abs. Over fifty, unless you're blessed by the right genes, it's nearly impossible.
See also: More Joy of Abs