The core of your body is its middle, its very center, and it is what keeps your body in a straight up-and-down position. It includes all of the muscles of your torso — those in your back and abdominals. Ballet dancers have very strong cores, while people who sit too much have weak cores because they don't have to hold up their own bodies. When you think of the differences in these two shapes — even discounting the difference in body weight — you might use the words “long” and “elegant” for the dancer and the words “saggy” and “flabby” for the person with little core strength.
Creating a strong core immediately gives you the appearance and feeling of being thinner and leaner because you stand properly. It has been shown to reduce injuries in other athletic activities and to improve athletic skills. Football players, baseball players, surfers, and skiers all do core strength exercises.