"Your core is upper and lower abs, internal and external obliques (ribcage and sides of waist) as well as upper and lower back muscles. Make sure you work all of them for proper core strengthening...
Many people work their abs by lying on the floor and doing crunches, but that only works your upper abs. Other exercises must be done for other areas of the core to have solid core strength.
Having a strong core all the way around helps you create a strong foundation for total body strength.
When you think core, think of the trunk of a tree. The branches of the tree are held up when the trunk of the tree is strong."
--from my book Real Health, Real Life.