What causes the noise when you crack a joint? Is joint cracking harmful? 
The exact mechanism is unclear. Here are some possible causes:


rapid collapsing of cavities inside the joint [1];
rapid ligament stretching [1];
breaking of intra-articular adhesions [1];
escaping gases from synovial fluid [2];
movements of joints, tendons and ligaments [2];
mechanic interaction between rough surfaces [2], mostly in pathological situations like arthritis (and it is called crepitus [3]).


There are no known bad effects of joint cracking [1, 4].


  There are no long term sequelae of these noises, and they do not lead to future problems. There is no basis for the admonition to not crack your knuckles because it can lead to arthritis. There are no supplements or exercises to prevent these noises [4].


And no good effects either:


  Knuckle "cracking" has not been shown to be harmful or beneficial. More specifically, knuckle cracking does not cause arthritis [5].




References:


Wikipedia contributors, "Cracking joints," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Cracking_joints&oldid=617403659 (accessed July 22, 2014).
The Library of Congress. Everyday Mysteries. What causes the noise when you crack a joint? Available from http://www.loc.gov/rr/scitech/mysteries/joint.html (accessed 22.07.2014)
Wikipedia contributors, "Crepitus," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Crepitus&oldid=615557134 (accessed July 22, 2014).
Johns Hopkins Sports Medicine Patient Guide to Joint Cracking & Popping. Available from http://www.hopkinsortho.org/joint_cracking.html (accessed 22.07.2014)
WebMD, LLC. Will Joint Cracking Cause Osteoarthritis? Available from http://www.webmd.com/osteoarthritis/guide/joint-cracking-osteoarthritis (accessed 22.07.2014)

