Tennis elbow and golfer's elbow are forms of elbow tendinitis caused by overuse and repetitive strain on the tendons in the forearm. Tennis elbow involves the tendons on the outside of the elbow and is more common, while golfer's elbow affects the inner tendons. Both result from repetitive motions like swinging, gripping, or flexing and can be treated with rest, anti-inflammatories, bracing, and physical therapy.