Pop... Pop... Pop...

What is popping AKA cracking? Like when someone pops their knuckles. I've heard numerous explanations, all kinds of stuff from air in joints to arthiritis. All seem designed to scare you into not doing it anymore by overprotective moms. 
PS: As a useless point, I can pop my knuckles, fingers*, elbows, neck, back and toes. I'm currently working on my knee.
*Figure it this way. The finger has three bones (as far as I can tell), one being at the tip, two in the middle, and three by the knucke. I can pop my finger between two and three.

PS: As a useless point, I can pop my knuckles, fingers*, elbows, neck, back and toes. I'm currently working on my knee.

*Figure it this way. The finger has three bones (as far as I can tell), one being at the tip, two in the middle, and three by the knucke. I can pop my finger between two and three.