To keep the joints from rubbing against eachother a material called cartilage is protecting the meeting point of the bones. In the case of osteoarthritis, the cartilage becomes too thin, causing changes in the joint.
What happens when you get osteoarthritis?
If the cartilage in the joints starts thinning, the joint will start changing, trying to fix the problem. Unfortunately these changes sometimes worsen the symptoms and causes stiffness and pain. The end of the bone affected can start to grow outwards, into little pikes called osteophytes. Extra liquid can be produced, causing swelling and the ligaments will try to stabilize the joint by contracting and thickening.
In severe cases of osteoarthritis, the cartilage will degenerate, causing the bones to rub against eachother and wearing them down.
In the long run osteoarthritis can cause wear and tear to both cartilage, ligaments and bones.
You can find more information at the Osteoarthritis Forum.
Source: Arthritis Research UK