The progestin released by the hormonal IUD prevents fertilization by making the mucus in the cervix thick and sticky, so sperm can't get through to the uterus. The progestin and IUD change the uterine lining, preventing implantation in the rare case that fertilization occurs.
The copper in the copper IUD is toxic to sperm. It causes changes in the uterus and fallopian tubes that kill sperm. The IUD changes the uterine lining, which prevents implantation in the rare case that fertilization occurs.