Yoga means union of mind and body and is a complete system or philosophy of life dating back several thousand years.
There are so many different kinds of yoga, and several thousand asanas - or poses, that it's difficult to know where to begin, but suffice to say, there is a kind of yoga for every person and every condition.
How Yoga uses concentration on the breath to connect mind and body and at the same time doing gentle exercises that stimulate various parts of the body and nervous system has shown to be very beneficial for all kinds of ailments, most notably depression - a common side effect of chronic pain.
You may think "I can't do that" when you see these yoga-health nuts twisting and bending in incredible positions, but that gymnastic style is just ONE form of yoga... a lot of yoga you can do while sitting still, or lying down, and you still get the great health benefits. My grandmother, who also suffered from Rheumatoid Arthritis, practiced yoga way into her eighties, and always claimed it had done wonders for her health.
Most important is that you find a well qualified instructor or yoga therapist, who understands your health problems and can tailor a home training program to your specific needs. With just a few exercises done regularly every day, you may soon be making significant improvements.