This is a long and very detailed article by Meghan Callaway on why and how to train the abs to be strong, functional, and aesthetically pleasing.
While a rock hard six-pack is aesthetically pleasing, the musculature of the core is important for providing the body with the pillar of strength and stability that is necessary to perform at a high level, stay healthy, and look amazing. The four layers of the abdominal wall have distinct anatomical variance and functions that need to be programmed in specific movements and functions in order to translate your training to aesthetic and functional results. Obviously a lean and chiseled core looks pretty damn sexy, but it also serves to improve posture and performance, making your body more resilient and bulletproofed in the process. Traditional core training programs are anything but effective, with many neglecting core stability, not tapping into the all important mind-muscle connection and aimlessly only training spinal flexion. Training the core in three distinct categories of movement, anti-extension, anti-rotation and anti-lateral flexion can produce the results you are seeking, both in the aesthetics department and translating into your big lifts and athletic performance. Use the 12 core training variations complete with coaching notes and video tutorials to improve your core training routine, and please, be better than just the crunch.