Iud is intrauterine device. They insert a .T. Shaped device up into your uterus. There are currently two on the market, paragard, and mirena.
Mirena is hormonal, and good for 5 years, paragard is non hormonal andgood for 10.
They both work by creating a hostile environment inside of your uterus. They kill sperm, and make your uterine lining inhospitable, so even if an egg is fertilized, it could not implant. You do still ovulate on both iud's.