Arthritis is a general term that refers to joint injury and damage. A joint is any area where two bones come together and can move against each other. For example, the knee joint is the area where the femur (the thigh bone) and the tibia (the leg bone) come together.
The most common type of arthritis is "osteoarthritis." This is usually caused by wear-and-tear. We will all have osteoarthritis if we live long enough.
People with rheumatoid arthritis can also develop joint injury and damage. However, the cause of the injury is different. In rheumatoid arthritis, the body mistakes its own cells for enemy cells (ex. bacteria, cancer cells, etc.). Therefore, it attacks its own cells. The body forms "antibodies" against its own joints, which leads to joint inflammation and damage.