This describes what happens when a person cracks their knuckle joints. All of the joints in the body are surrounded by connective tissues, ligaments and synovial fluid. Synovial fluid is a clear, thick liquid that contains dissolved gasses. When joints, such as the ones between finger bones are pulled apart as a person cracks his or her knuckles, the volume increases between the joints, lowering the pressure inside the synovial fluid and causing the gases within it to burst. This causes a popping sound.
Joints that have been cracked cannot be popped again immediately, as it takes roughly 25 minutes before gases are dissolved back into the synovial fluid again.
So far, no studies have found that knuckle cracking causes any damage in finger joints or contributes to the development of arthritis. However, some studies have shown that repetitive knuckle-cracking may affect the soft tissue surrounding the joint and decrease the grip strength of a person’s hand.
BBC, Scientific American, How Stuff Works