When a person "pops" their fingers, back etc., what is actually happening to cause the cracking sound, and are there any long term effects?
When you "pop" your joints you are temporarily displacing them out of their normal position. The pressure in synovial fluid (essentially a lubricant) in between the bones is reduced when you do this. As a result bubbles form, expand, and then burst creating the popping sound that you hear. It is a process called cavitation - air bubbles forming in a fluid. The reason you can't pop your joints for a while after this is the gas you've released the first time needs to be dissolved back into the synovial fluid. Once this happens you can crack them again. Contrary to popular belief, no serious long term damage has been found in any research on chronic joint poppers.
Another possible cause of the popping sound is your muscles, like say for instance when you crack your neck. Contrary to popping your joints, doing things like cracking your neck can cause permanent long term damage because you are stretching the ligaments. It's a viscous cycle because cracking your neck loosens the muscles and makes you feel temporarily better, but then they get even tighter to compensate for the loosened ligaments. Repeat cycle.