Arthritis is an inflammation in a joint, while tendonitis is inflammation of a tendon, the gristle-like cable that links muscle and bone. It can get confusing because a tendon is one component of the complex structure of a joint, though it’s not inside the joint itself. Overuse or repetitive stress – like lots of tennis or jogging – can lead to either arthritis, tendonitis or both, depending on where the damage occurs.