The rectus abdominis is the muscle you think of when you think "abs." It's the outermost abdominal muscle, and runs vertically along each side of your abdominal wall. The transverse abdominis is the deepest muscle of the abdominal wall, which means it's closest to your spine, and basically wraps around your torso between your ribs and your hips. The oblique muscles run along the sides of your torso, and there are two sets: internal and external. The internal obliques lie above the transverse abdominis, and then the external obliques are on top of those (they're the most superficial of the bunch). There's also a handful of other smaller muscles in this area—what we call the core—that work to stabilize the spine and allow us to bent and twist and lift without hurting ourselves.
Ah the six-pack. The goal of most every fitness enthusiast. A tight, lean, shredded stomach not only looks great, but also feels great and builds more confidence. Let's face it, a chiseled midsection is something we all strive to strut. Unfortunately, not everyone is willing to put in the time and effort to build this work of art. For those that do however, the rewards are well worth the struggle.