In a nutshell, osteoarthritis is wear-and-tear on the joint. It can happen at any age, not just to "old people". It can be brought on by an injury, a congenital deformity, overuse in a sport or job.
RA is a malfunction of the immune system in which it mistakes joint tissue, mainly the cartilage, as something that must be attacked and eliminated. This causes the typical swelling, redness, heat, and over time, damage that causes pain and disability.