Carpal tunnel syndrome occurs when the median nerve running from the forearm into the hand gets compressed by the transverse carpal ligament at the wrist. The median nerve travels through a small canal within the palm side of the wrist to enter the hand. This canal is made up of bone, tendons and ligaments. With repetitive wrist movement or constant wrist pressure such as with supermarket checkers or typists, this canal can become inflamed, which in turn compresses the median nerve. Patients with carpal tunnel syndrome may experience symptoms of numbness and tingling in the hand, wrist pain, or grip weakness. 

Source: medicinenet.com