Your wife's was probably a hormonal one.
There are 3 basic types.
Copper - These IUDs are covered in coils of copper wire. The copper acts as a spermicide and also provokes the immune system making the uterine environment unfavourable for fertilization. They last from 5 to 12 years depending on design. As De thuong said, they tend to result in heavier and more painful menstruation. Failure rate is 0.7-0.8% per year, making them one of the best methods.
Hormonal - These have a hormone releasing medium. They work about the same as any other hormonal contraceptive, but the hormone is applied topically rather than systemically, allowing for much lower dosing (tens of micrograms per day) making it more reliable and less prone to side effects. Unlike copper, these usually result in reduced menstrual flow, or none at all in some. They last 3-7 years. Failure rate is about 0.2%.
Inert - Oddball type that was used in Asia in the 80s, 90s, and 00s and were very common in China. These are similar to copper IUDs, but used stainless steel instead. They work the same as the copper ones, but without the spermicidal effect. Same side effects as copper too, but not as effective (~5% failure rate). They're no longer made (replaced with copper ones), but some are still in use among Chinese women, particularly expats, where they confuse doctors when they want them removed.