Genuine parts are specifically designed and tested to meet quality specifications. Non-genuine parts are made by factories that are manufactured to be as cheap as possible, often using inferior materials and workmanship. Used parts may include genuine parts removed from cars.
Even though a second-hand part might have originally been a genuine part, its structural integrity may have been compromised, it may show signs of wear and tear, or it may be damaged in a way that's not visibly noticeable. These factors can all lead to major problems and even genuine used parts may pose a safety risk.