Tendons are the tough bits of gristle that join muscles on to bone.
When the muscle contracts it pulls on the tendon and results in movement of the bone.
Tendonitis is the term used for inflammation in a tendon. Anything ..."itis" means inflammation in medical terminology. So for example, dermatitis is inflamed skin, appendicitis is an inflamed appendix, tonsillitis is inflamed tonsils and so on.
Around the shoulder the inflammation is normally found near to the point where tendons join on to the bone. Tendonitis of the shoulder usually reflects overuse of the arm although it can sometimes start suddenly after injury.
Sometimes shoulder tendonitis is found in association with a tear in the rotator cuff muscles.