There are various types of RSI. These are, as the name suggests, caused by cumulation of repetitive movements which may be large, such as those involved in sports, or small, such as typing, use of a computer mouse, or tool use.
Two of the most common forms of RSI are lateral and medial epicondylitis. Lateral epicondylitis or tennis elbow, does not commonly, despite it's name, result from playing tennis. The majority of cases are among office workers/computer users, or other people who make small repetitive movements, e.g. screwdriver, or other tool, use.
The lateral epicondyle is a bony prominence of the humerus (upper arm bone), and is located, if you stand with your arms at your side with the palms of your hands facing forward, on the outside of your elbow. The muscles which act to extend the wrist and fingers attach, via tendons, to the lateral epicondyle. Degeneration of these tendons can occur, particularly (due to poor blood supply) near the point which they attach to bone. This results in pain in the area of the lateral epicondyle, and, often, forearm, and possibly reduced grip strength. Often there is involvement of the shoulder region, particularly of one of the biceps tendons.
Medial epicondylitis is a similar condition, but with the pain affecting the medial epicondyle on the inside of the elbow, and affecting the muscles which flex the fingers and wrist.
Other forms of RSI include Thoracic Outlet Syndrome (TOS), Carpal Tunnel Syndrome (CTS), various forms of tendinosis (degeneration of a tendon without inflammation), and, less commonly, tendinitis (tendon injury with inflammation).
Picture-Lateral Epicondylitis/Tennis Elbow (source BruceBlaus)