Carpal tunnel syndrome (CTS) occurs when the median nerve, which runs from the forearm into the palm of the hand, becomes pressed or squeezed at the wrist. The carpal tunnel—a narrow, rigid passageway of ligament and bones at the base of the hand—houses the median nerve and the tendons that bend the fingers. The median nerve provides feeling to the palm side of the thumb and to the index, middle, and part of the ring fingers (although not the little finger). It also controls some small muscles at the base of the thumb.
Source : US National Institute of Neurological Disorders and Stroke
Most instances of Carpal Tunnel Syndrome are 'idiopathic' that's to say 'of unknown cause'.
'Contributing factors' - i.e. factors which make it more likely to develop CTS - include trauma or injury to the wrist, an overactive pituitary gland, an underactive thyroid gland, rheumatoid arthritis, repeated use of vibrating hand tools, fluid retention during pregnancy or menopause, or the development of a cyst or tumor in the carpal tunnel.
It's often associated with, but is not the same as RSI. Estimates are that around 5% of the population suffers from the condition.