Tendons are cords of tissue that anchor muscles to bones. They slide back and forth as our muscles contract and our joints flex. To prevent chafing and to keep them in position, the tendons are enclosed in special coverings (sheaths) that are lubricated. When something goes wrong that prevents the tendon from moving smoothly, pain and stiffness result.
When tendons are damaged and inflamed, the condition is commonly known as tendinitis. If the problem is in the lining of the tendon's sheath, it's called tenosynovitis.