Osteoarthritis is a degenerative joint disease where the protective cartilage in joints wears down over time, causing pain, stiffness, and swelling. It most commonly affects the hands, knees, hips, and spine and can be caused by factors like age, overuse, injury, and being overweight. While there is no cure, symptoms can be managed with treatments such as weight management, exercise, and medication, and surgery may be an option for severe cases.