Rectus Abdominus Flex the spine (bringing the rib cage closer to the pelvis). This is seen in the abdominal crunching movement. When the movement is reversed, the Rectus Abdominus acts to bring the pelvis closer to the rib cage (e.g. with a leg raise movement).

Transverse Abdominus Acts as a natural weight belt, keeping your insides in. This muscle is essential for trunk stability as well as keeping your waist tight.
Internal and External Obliques Work to rotate the torso and stabilize the abdomen.