Chiropractic is a form of alternative medicine that emphasizes diagnosis, treatment and prevention of mechanical disorders of the musculoskeletal system, especially the spine, under the hypothesis that these disorders affect general health via the nervous system. Chiropractic treatment technique primarily involves manual therapy, including manipulation of the spine and other joints, as well as soft tissue therapies; treatment also commonly includes exercise and health and lifestyle counseling.
Chiropractic adjustment — also called spinal manipulation — is the most common treatment used in chiropractic care. Spinal manipulation is a passive manual maneuver during which a three-joint complex is taken past the normal range of movement, but not so far as to dislocate or damage the joint; its defining factor is a dynamic thrust, which is a sudden force that causes an audible release and attempts to increase a joint's range of motion. More generally, spinal manipulative therapy describes techniques where the hands are used to manipulate, massage, mobilize, adjust, stimulate, apply traction to, or otherwise influence the spine and related tissues. You often will hear a popping or cracking noise during chiropractic adjustment as the joint is manipulated.
Chiropractic care is an outgrowth of belief in the concept that the body has a natural and powerful ability to heal itself, that the body's structure (bones, joints and muscles) and capacity for healthy function are closely intertwined, and that chiropractic treatment helps balance the body's structure and function and promotes self-healing.
More information available at: Wikipedia.