Having a strong core is not just about having a defined "six-pack", it is made up of many muscles that run the entire length of the torso. Like the core of an apple, it is the center area of the body that supports the spine. It is made up of the muscles of the trunk, extending from the base of your head to your pelvis. They include:
Transverse abdominus (on each side of the belly button)
Internal and external obliques (run diagonally from the ribs to pelvis)
Rectus abdominus (six-pack muscles)
Multifidus and erector spinae (run along spine from head to pelvis)
Developing and maintaining a strong core is important for many reasons. It is the connection between the upper and lower extremities and it serves to transfer power between the two. It is the center of gravity that allows you to move (examples: walking, sitting, and running). The core teaches the muscles of the trunk to work together efficiently and effectively in order to improve balance and stability which can prevent falls and injury. Benefits of improving core strength are many including improved posture, better balance, reduced back pain, and improved breathing.
