An IUD is a small, T-shaped contraceptive device that is inserted into the uterus to prevent pregnancy. There are two main types: hormonal IUDs, which release progestin to thicken cervical mucus and suppress ovulation, and copper IUDs, which create an inhospitable environment for sperm. IUDs are highly effective, long-acting, and reversible forms of birth control that can last anywhere from 3 to 10 years depending on the type.