Approximate effectiveness: 98-99%
No protection against STDs
An IUD is a small T-shaped plastic device inserted into the vagina by a health care practitioner, usually a nurse or doctor. (Copper IUDs have been in use for a long time but have caused problems for many of its users.) An IUD generally prevents sperm from reaching an egg and if one does and an egg becomes fertilised, the IUD will likely prevent the egg from attaching itself to the wall of the uterus (the wall is also called the endometrium if you prefer big words!). At least one type called a Mirena IUD also releases hormones which help prevent an egg from being released. An IUD is a fairly complicated procedure compared to some other birth control methods and can be quite expensive. IUDs have been associated with some medical problems like possible tearing of the uterus when it is being inserted, heavier and longer periods, spotting between periods, PID (Pelvic Inflammatory Disease) and sometimes heavy cramping. Anecdotally (stories told by users) some users also complain of weeks of depression, of headaches, heartburn and other internal problems. Anectdotal stories are not scientifically examined but if you are considering an IUD, at least do an internet search for "IUD, Mirena (if that is the one you are thinking of using ) and side effects".
Since there are so many easier to use, less expensive birth control methods, with fewer side effects, you should consult with your doctor in detail about the possible use of an IUD. Your doctor or gynecologist will have to fit you for it as well.