The most common form of arthritis is osteo-arthritis. This is a slowly progressive, degenerative joint disease that results from the breakdown of cartilage and leads to pain and stiffness. This will usually affects the knees, hips, hand and spine.
The prevalence is 10-20% of the adult population and in the US it is estimated that we spend 1 billion annually on joint surgery primarily from this a year.
There are many causes, but the primary reason is mechanical. This is from a bony misalignment in the joint, sometimes from trauma. After time the joint will rub and wear down on itself and after time the arthritis or decay process will begin. Much like a cavity is decay in teeth.
Arthritis will create inflammation, which will also irritate nerves and cause more pain. Which is why many people feel relief from taking arthritis treatment Ibprofen or some other kind of NSAID. Long term use of these are quite damaging, so it is important to consult your doctor on a safer long term method to decrease inflammation in your body.

