The IUD is an acronym for Intra-Uterine Device. A form of contraception.
It is usually a piece of metal (usually copper) with some plastic attached that is stuffed into the woman's uterus and is usually inserted by a gynecologist.
IUDs work by causing spontaneous abortions of conceived zygotes.
One downside of IUDs are that if pregnancy does occur despite the IUD being present, the results are often not pretty. The other major thing that could go wrong is that the IUD itself could perforate the uterus. Ouch.
Use the pill - it's safer.