What is the difference between arthritus and tendonitus?

Joints vs muscles: Arthritis is a general term for impairment of a joint such as a knee, which can be inflammatory (rheumatoid arthritis) or degenerative (osteoarthritis). Tendinitis (also called tendinosis) is a problem affecting a tendon or tendons, which attach muscle to bone. The difference is not just anatomy but also the causes can be very different.

Well,: Arthritis is the inflammation of the joints, tendinitis is the inflammation of tendons through which the muscles attach to the bone. Tendons are sliding in a sheet and in case of excessive efforts inflammation can set and the gliding of the tendon in its sheet becomes very painful, the best example is an ankle sprain: a wrong movement, excessive forces and the tendons flare up.