An IUD, or intrauterine device, is a small, T-shaped contraceptive device that a healthcare provider places inside a woman's uterus. It works by preventing sperm from fertilizing an egg and can also inhibit implantation. IUDs are highly effective, with a success rate of over 99%, and can last several years, depending on the type. There are two main types of IUDs: hormonal and copper. Hormonal IUDs release hormones to prevent ovulation, while copper IUDs use copper to create an environment that is toxic to sperm. Both types are reversible, meaning fertility returns quickly after removal.