Just as our skin is a barrier that protects our inner organs from the outside world, the lungs are protected by a special lining called the pleura. This lining includes the tissue inside the chest wall and tissue that surrounds the lungs. It protects the lungs from the chest wall and allows them to easily slide against each other.
Although it is normally smooth, if this lining becomes inflamed, the surface of the pleura becomes rough and irritated, causing the condition known as pleurisy. Sometimes, a collection of fluid around the lungs may occur - this is called wet pleurisy. Without the buildup of fluid between the lungs and the lining, the condition is known as dry pleurisy.