What's the difference between tendonitis and arthritis?

No, can be the same! Tendons attach to the joint capsule or bone. Tendinitis is a basic form of enthesophy (which also includes ligamentous and periosteal inflammation) is basic feature of the most common inflammatory arthopathies, spondyloarthritis. Inflammation of tendon sheath is common in ra.
Tendonitis. Tendonitis is inflammation of a tendon. A tendon is the part of the muscle that attaches to the bone. An example would be tendonitis of a rotator cuff tendon in the shoulder. Arthritis is inflammation in a joint. Examples would be osteoarthritis of the knee. Hope this helps.
Tendon not joint. Arthritis by definition is inflammation in the joint. The tendon helps attach muscles to the bone. When a tendon is inflammed, swollen, and tender it is called tendnonitis. Both are describing inflammation but at different locations: the joint verses the tendon.
Completely different. Tendinitis is inflammation around the muscles and tendons that may cause pain, often from overuse or compensating for a weak muscle. Arthritis is defined by inflammation within a joint such as the knee. Osteoarthritis is an example of arthritis where the cartilage cushion is worn down, causing pain and stiffness.