An intrauterine device (IUD) is a form of long-term birth control that can last for 5 to 8 years. There are two types of IUD; the older copper IUD and the newer hormone-releasing IUD. Both are inserted into the uterus and work to prevent pregnancy by preventing sperm from moving through the uterus. The copper IUD is toxic to sperm as is the hormone released from the hr-IUD. They also cause changes to the lining composition of the uterus so if an egg is fertilised it is unable to implant.
For more information see Intrauterine Device (IUD)