What is the main difference between arthritis and some other illness?

Depends. Arthritis just means inflammation of the joints and there are many types so it depends on what disease you are comparing it to.
JOINT PAIN. Arthritis refers to inflammation of the joints. People with arthritis experience joint pain and stiffness. Some types of arthritis are part of an autoimmune disease. In this case there may be other symptoms such as fatigue, fever, and rashes.