The truth is that we all have the same basic structure to our abdominals, which are often confused with the stomach. The stomach digests your food, but if you're not careful your abdominals will wear your food. That's what you see when a man or woman has that six-pack look. The reason you can see their definition is because they have reduced the fat in front of the muscle and developed musculature that is the envy of the masses.
Many people refer to the upper and lower abs.
These are just the areas in which the muscle is located, toward the upper and lower ends of the same muscle. Your rectus abdominus is one muscle and should always be considered as such. Specific exercises can be performed to enlist the action from either the top or the bottom part of the muscle. The sides are another group, called obliques. By Jon Giswold from Basic Training .